switch to flake8 to be able to suppress warnings

there will be tons of pyflakes warnings from the python3 porting strategy

setup.cfg

@@ -63,3 +63,10 @@ versionfile_source = src/_zkapauthorizer/_version.py
 versionfile_build = _zkapauthorizer/_version.py
 tag_prefix = release-
 parentdir_prefix = ZKAPAuthorizer
+
+[flake8]
+# Enforce all pyflakes constraints, and also prohibit tabs for indentation.
+# Reference:
+#   https://flake8.pycqa.org/en/latest/user/error-codes.html
+#   https://pycodestyle.pycqa.org/en/latest/intro.html#error-codes
+select = F, W191

tests.nix

@@ -51,9 +51,9 @@ let
       mkdir -p $out
 
       pushd ${zkapauthorizer.src}
-      ${python}/bin/pyflakes src
       ${lint-python}/bin/black --check src
       ${lint-python}/bin/isort --check src
+      ${lint-python}/bin/flake8 src
       popd
 
       ZKAPAUTHORIZER_HYPOTHESIS_PROFILE=${hypothesisProfile'} ${python}/bin/python -m ${if collectCoverage