dev: Avoid pytest 7.0.0 on Python 3.9

It contains a fatal bug¹ when used on 3.9 because it incorrectly guards
a 3.10-only implementation detail with a >=3.9 version check, which
results in:

    ModuleNotFoundError: No module named 'importlib.readers'

when trying to load/parse Python modules.

Pytest 7.0.0 is only ~11 hours old at the moment, and another person
filed the upstream bug report as I was writing my own.  What a life,
living on the edge. 😬

Adjusts the warnings filter slightly in pytest.ini because
PytestRemovedIn8Warning is only present in pytest 7 and for now our
tests on 3.9 will use pytest 6.

¹ pytest-dev/pytest#9608

pytest.ini

@@ -14,4 +14,4 @@ filterwarnings =
     error
     ignore:SelectableGroups dict interface:DeprecationWarning:flake8
     ignore:<class 'pytest_flake8[.]Flake8Item'> is not using a cooperative constructor:pytest.PytestDeprecationWarning
-    ignore:The .+? argument to Flake8Item is deprecated:pytest.PytestRemovedIn8Warning
+    ignore:The .+? argument to Flake8Item is deprecated:pytest.PytestDeprecationWarning

setup.py

@@ -122,7 +122,8 @@ def find_namespaced_packages(namespace):
             "flake8",
             "mypy",
             "nextstrain-sphinx-theme",
-            "pytest",
+            "pytest; python_version != '3.9'",
+            "pytest !=7.0.0; python_version == '3.9'",
             "pytest-flake8",
             "recommonmark",
             "sphinx>=3",