Run Python 3.10 under coverage (#818)

Since the matching code is Python 3.10-specific, we run it under coverage.

.github/workflows/main.yml

@@ -13,12 +13,12 @@ jobs:
     name: "Python ${{ matrix.python-version }}"
     runs-on: "ubuntu-latest"
     env:
-      USING_COVERAGE: "2.7,3.7,3.8"
+      USING_COVERAGE: "2.7,3.7,3.8,3.10.0-beta - 3.10"
 
     strategy:
       fail-fast: false
       matrix:
-        python-version: ["2.7", "3.5", "3.6", "3.7", "3.8", "3.9", "3.10.0-alpha - 3.10", "pypy2", "pypy3"]
+        python-version: ["2.7", "3.5", "3.6", "3.7", "3.8", "3.9", "3.10.0-beta - 3.10", "pypy2", "pypy3"]
 
     steps:
       - uses: "actions/checkout@v2"
@@ -40,7 +40,7 @@ jobs:
       # parsing errors in older versions for modern code.
       - uses: "actions/setup-python@v2"
         with:
-          python-version: "3.9"
+          python-version: "3.10.0-beta - 3.10"
 
       - name: "Combine coverage"
         run: |

conftest.py

@@ -4,6 +4,8 @@ import sys
 
 from hypothesis import HealthCheck, settings
 
+from attr._compat import PY310
+
 
 def pytest_configure(config):
     # HealthCheck.too_slow causes more trouble than good -- especially in CIs.
@@ -23,9 +25,9 @@ if sys.version_info[:2] < (3, 6):
             "tests/test_next_gen.py",
         ]
     )
-if sys.version_info[:2] < (3, 10):
+if not PY310:
     collect_ignore.extend(["tests/test_pattern_matching.py"])
-if sys.version_info[:2] >= (3, 10):
+if PY310:
     collect_ignore.extend(
         [
             "tests/test_mypy.yml",

src/attr/_compat.py

@@ -8,6 +8,7 @@ import warnings
 
 PY2 = sys.version_info[0] == 2
 PYPY = platform.python_implementation() == "PyPy"
+PY310 = sys.version_info[:2] >= (3, 10)
 
 
 if PYPY or sys.version_info[:2] >= (3, 6):

tox.ini

@@ -61,10 +61,21 @@ extras = {env:TOX_AP_TEST_EXTRAS:tests}
 commands = coverage run -m pytest {posargs}
 
 
+[testenv:py310]
+# Python 3.6+ has a number of compile-time warnings on invalid string escapes.
+# PYTHONWARNINGS=d and --no-compile below make them visible during the Tox run.
+basepython = python3.10
+install_command = pip install --no-compile {opts} {packages}
+setenv =
+    PYTHONWARNINGS=d
+extras = {env:TOX_AP_TEST_EXTRAS:tests}
+commands = coverage run -m pytest {posargs}
+
+
 [testenv:coverage-report]
-basepython = python3.7
+basepython = python3.10
 skip_install = true
-deps = coverage[toml]>=5.0.2
+deps = coverage[toml]>=5.4
 commands =
     coverage combine
     coverage report