diff --git a/.github/workflows/check.yml b/.github/workflows/check.yml
index e15936d..54a78bb 100644
--- a/.github/workflows/check.yml
+++ b/.github/workflows/check.yml
@@ -64,6 +64,7 @@ jobs:
       fail-fast: false
       matrix:
         tox_env:
+          - type
           - dev
           - readme
     steps:
diff --git a/src/pipdeptree/_discovery.py b/src/pipdeptree/_discovery.py
index 591bb35..1cce91e 100644
--- a/src/pipdeptree/_discovery.py
+++ b/src/pipdeptree/_discovery.py
@@ -2,12 +2,6 @@ from __future__ import annotations
 
 from typing import TYPE_CHECKING
 
-try:  # noqa: SIM105
-    from pip._internal.operations.freeze import FrozenRequirement  # noqa: F401
-except ImportError:
-    pass  # type: ignore[attr-defined, no-redef]
-
-
 if TYPE_CHECKING:
     from pip._vendor.pkg_resources import DistInfoDistribution
 
diff --git a/src/pipdeptree/_render/__init__.py b/src/pipdeptree/_render/__init__.py
index bcbc30b..da3e058 100644
--- a/src/pipdeptree/_render/__init__.py
+++ b/src/pipdeptree/_render/__init__.py
@@ -10,9 +10,10 @@ from .text import render_text
 
 if TYPE_CHECKING:
     from pipdeptree._cli import Options
+    from pipdeptree._models import PackageDAG
 
 
-def render(options: Options, tree: PackageDAG) -> None:  # noqa: F821
+def render(options: Options, tree: PackageDAG) -> None:
     if options.json:
         print(render_json(tree))  # noqa: T201
     elif options.json_tree:
@@ -20,6 +21,7 @@ def render(options: Options, tree: PackageDAG) -> None:  # noqa: F821
     elif options.mermaid:
         print(render_mermaid(tree))  # noqa: T201
     elif options.output_format:
+        assert options.graph_output is not None  # noqa: S101
         render_graphviz(tree, output_format=options.graph_output, reverse=options.reverse)
     else:
         render_text(tree, options.depth, options.encoding_type, options.all, options.freeze)
diff --git a/src/pipdeptree/_render/mermaid.py b/src/pipdeptree/_render/mermaid.py
index 1d96db4..f5b52c7 100644
--- a/src/pipdeptree/_render/mermaid.py
+++ b/src/pipdeptree/_render/mermaid.py
@@ -1,23 +1,15 @@
 from __future__ import annotations
 
 from textwrap import dedent
-from typing import TYPE_CHECKING
+from typing import TYPE_CHECKING, Final
 
 from pipdeptree._models import ReversedPackageDAG
 
 if TYPE_CHECKING:
     from pipdeptree._models import PackageDAG
 
-
-def render_mermaid(tree: PackageDAG) -> str:  # noqa: C901
-    """
-    Produce a Mermaid flowchart from the dependency graph.
-
-    :param tree: dependency graph
-    """
-    # List of reserved keywords in Mermaid that cannot be used as node names.
-    # See: https://github.com/mermaid-js/mermaid/issues/4182#issuecomment-1454787806
-    reserved_ids: set[str] = {
+_RESERVED_IDS: Final[frozenset[str]] = frozenset(
+    [
         "C4Component",
         "C4Container",
         "C4Deployment",
@@ -38,7 +30,19 @@ def render_mermaid(tree: PackageDAG) -> str:  # noqa: C901
         "linkStyle",
         "style",
         "subgraph",
-    }
+    ],
+)
+
+
+def render_mermaid(tree: PackageDAG) -> str:  # noqa: C901
+    """
+    Produce a Mermaid flowchart from the dependency graph.
+
+    :param tree: dependency graph
+    """
+    # List of reserved keywords in Mermaid that cannot be used as node names.
+    # See: https://github.com/mermaid-js/mermaid/issues/4182#issuecomment-1454787806
+
     node_ids_map: dict[str, str] = {}
 
     def mermaid_id(key: str) -> str:
@@ -48,7 +52,7 @@ def render_mermaid(tree: PackageDAG) -> str:  # noqa: C901
         if canonical_id is not None:
             return canonical_id
         # If the key is not a reserved keyword, return it as is, and update the map.
-        if key not in reserved_ids:
+        if key not in _RESERVED_IDS:
             node_ids_map[key] = key
             return key
         # If the key is a reserved keyword, append a number to it.
diff --git a/tests/conftest.py b/tests/conftest.py
index 0f26518..b38d427 100644
--- a/tests/conftest.py
+++ b/tests/conftest.py
@@ -33,7 +33,7 @@ def mock_pkgs() -> Callable[[MockGraph], Iterator[Mock]]:
 
 @pytest.fixture()
 def example_dag(mock_pkgs: Callable[[MockGraph], Iterator[Mock]]) -> PackageDAG:
-    packages = {
+    packages: MockGraph = {
         ("a", "3.4.0"): [("b", [(">=", "2.0.0")]), ("c", [(">=", "5.7.1")])],
         ("b", "2.3.1"): [("d", [(">=", "2.30"), ("<", "2.42")])],
         ("c", "5.10.0"): [("d", [(">=", "2.30")]), ("e", [(">=", "0.12.1")])],
diff --git a/tests/render/test_mermaid.py b/tests/render/test_mermaid.py
index 7706481..e59e352 100644
--- a/tests/render/test_mermaid.py
+++ b/tests/render/test_mermaid.py
@@ -4,7 +4,7 @@ from textwrap import dedent, indent
 from typing import TYPE_CHECKING, Callable, Iterator
 
 from pipdeptree._models import PackageDAG
-from pipdeptree._render import render_mermaid
+from pipdeptree._render.mermaid import render_mermaid
 
 if TYPE_CHECKING:
     from unittest.mock import Mock
diff --git a/tests/render/test_text.py b/tests/render/test_text.py
index 49d8245..220d9db 100644
--- a/tests/render/test_text.py
+++ b/tests/render/test_text.py
@@ -5,7 +5,7 @@ from typing import TYPE_CHECKING
 
 import pytest
 
-from pipdeptree._render import render_text
+from pipdeptree._render.text import render_text
 
 if TYPE_CHECKING:
     from pipdeptree._models import PackageDAG
diff --git a/tests/test_models.py b/tests/test_models.py
index a3f30ce..8660682 100644
--- a/tests/test_models.py
+++ b/tests/test_models.py
@@ -65,7 +65,7 @@ def test_package_dag_filter(example_dag: PackageDAG) -> None:
 
 @pytest.fixture(scope="session")
 def t_fnmatch(mock_pkgs: Callable[[MockGraph], Iterator[Mock]]) -> Any:
-    graph = {
+    graph: MockGraph = {
         ("a.a", "1"): [("a.b", []), ("a.c", [])],
         ("a.b", "1"): [("a.c", [])],
         ("b.a", "1"): [("b.b", [])],