Enable flake8-pyi plugin to automatically include *.pyi files

This simplifies running flake8 tests and reduces the amount of F821 errors
reported (flake8-pyi enables support for forward references in *.pyi files).
The error code is left disabled until I clean up the remaining issues.

tests/flake8_test.py

@@ -1,47 +0,0 @@
-#!/usr/bin/env python3
-import pathlib
-import subprocess
-import sys
-
-PY2_ONLY_KEYWORDS = [
-    'StandardError',
-    'apply',
-    'basestring',
-    'buffer',
-    'cmp',
-    'coerce',
-    'execfile',
-    'file',
-    'intern',
-    'long',
-    'raw_input',
-    'reduce',
-    'reload',
-    'unichr',
-    'unicode',
-    'xrange',
-]
-
-root = pathlib.Path(__file__).parent.parent
-paths = list(sorted(str(p) for p in root.glob('**/*.pyi')))
-window = 0
-size = 100
-returncode = 0
-
-print('Running flake8 on {} .pyi files...'.format(len(paths)))
-
-while True:
-    chunk = paths[window:window + size]
-    if not chunk:
-        break
-
-    proc = subprocess.run(
-        ['flake8', '--builtins=' + ','.join(PY2_ONLY_KEYWORDS)] + chunk,
-    )
-    if proc.returncode:
-        print('flake8 run failed!')
-        sys.exit(1)
-
-    window += size
-
-print('flake8 run clean.')

tests/mypy_test.py

@@ -126,7 +126,7 @@ def main():
             flags.append('--strict-optional')
             if (major, minor) >= (3, 6):
                 flags.append('--fast-parser')
-            ##flags.append('--warn-unused-ignores')  # Fast parser and regular parser disagree.
+            # flags.append('--warn-unused-ignores')  # Fast parser and regular parser disagree.
             sys.argv = ['mypy'] + flags + files
             if args.verbose:
                 print("running", ' '.join(sys.argv))