lint: sync flake8 excludes from gitignore (#3029)

Author: gnprice

setup.cfg

@@ -2,13 +2,15 @@
 max-line-length = 99
 # typeshed and unit test fixtures have .pyi-specific flake8 configuration
 exclude =
-  # .gitignore'd directories
-  build/*,
-  docs/build/*,
-  out/*,
-  .venv/*,
-  .cache/*,
-  .mypy_cache/*,
+  # from .gitignore: directories, and file patterns that intersect with *.py
+  build,
+  @*,
+  env,
+  docs/build,
+  out,
+  .venv,
+  .mypy_cache,
+  .cache,
   # Sphinx configuration is irrelevant
   docs/source/conf.py,
   # external library with incompatible style