Pin coverage to <5.0 for now

Getting

```
Coverage.py warning: Module attr was previously imported, but not measured (module-not-measured)
```

and missing coverage for some reason. Google is not helpful.

setup.py

@@ -39,7 +39,7 @@ INSTALL_REQUIRES = []
 EXTRAS_REQUIRE = {
     "docs": ["sphinx", "zope.interface"],
     "tests": [
-        "coverage",
+        "coverage<5.0",
         "hypothesis",
         "pympler",
         "pytest>=4.3.0",  # 4.3.0 dropped last use of `convert`

tox.ini

@@ -50,7 +50,7 @@ commands = coverage run --parallel -m pytest {posargs}
 [testenv:coverage-report]
 basepython = python3.7
 skip_install = true
-deps = coverage
+deps = coverage<5.0
 commands =
     coverage combine
     coverage report