limit the virtualenv version in python3.4

See merge request pycqa/flake8!456

add video tutorial to plugin development page

See merge request pycqa/flake8!453

fix a few small spelling issues

See merge request pycqa/flake8!452

found via `pre-commit try-repo https://github.com/codespell-project/codespell --all-files`

fix skipping of physical checks when file does not end in newline

See merge request pycqa/flake8!451

Fix options docs to correct some defaults

See merge request pycqa/flake8!450

Commit f963641e changed some defaults,
but did not update the docs.

Support linting when missing sem_open syscall

See merge request pycqa/flake8!448

Platforms such as Termux on Android, and other exotic devices
do not provide a sem_open implementation on the OS level. This
is problematic, as the error resulting from this occurs when
calling multiprocessing.Pool, throwing an unhandled ImportError.

The issue itself is outlined in https://bugs.python.org/issue3770.

This change allows devices missing this system call to respond
to the missing feature by falling back to synchronous execution,
which appears to be the default behaviour if the multiprocessing
module is not found.

This change also adds a potential fix for developers working
on platforms where multiprocessing itself cannot be imported.
The existing code would set the name referencing the import to
None, but there are no clear checks to ensure this does not
result in an AttributeError later when multiprocessing.Pool
has accession attempts.

Existing users should see no difference in functionality, as they
will assumably already be able to use flake8, so will not be
missing this sem_open call.

Users on devices without the sem_open call will now be able
to use flake8 where they would be unable to before due to
unhandled ImportErrors.

upgrade black and run via pre-commit

See merge request pycqa/flake8!449

fix link in configuration docs

See merge request pycqa/flake8!447

add note to configuration.rst that inline comments are not supported

Closes #682

See merge request pycqa/flake8!446

Fixed misprint.

See merge request pycqa/flake8!445

Update registering-plugins.rst

See merge request pycqa/flake8!444

Tests: Add a value to mocked entry_points, so pytest 6+ can log it

See merge request pycqa/flake8!443

Pytest 6.0.0rc1 attempts to log a repr of our mocked entry_point,
but it fails, becasue the repr metohod expects a valua attribute:

Reproducer:

    $ tox -e py38 --force-dep 'pytest==6.0.0rc1'
    ...
    Traceback (most recent call last):
      File "/usr/lib/python3.8/logging/__init__.py", line 1081, in emit
        msg = self.format(record)
      File "/usr/lib/python3.8/logging/__init__.py", line 925, in format
        return fmt.format(record)
      File "/usr/lib/python3.8/logging/__init__.py", line 664, in format
        record.message = record.getMessage()
      File "/usr/lib/python3.8/logging/__init__.py", line 369, in getMessage
        msg = msg % self.args
      File "/tmp/flake8/.tox/py38/lib/python3.8/site-packages/flake8/plugins/manager.py", line 44, in __repr__
        self.name, self.entry_point.value
      File "/tmp/flake8/.tox/py38/lib/python3.8/site-packages/mock/mock.py", line 632, in __getattr__
        raise AttributeError("Mock object has no attribute %r" % name)
    AttributeError: Mock object has no attribute 'value'

skip sphinx 3.1.0

See merge request pycqa/flake8!438

See https://github.com/sphinx-doc/sphinx/issues/7807

Doctest documentation unclear

Closes #659

See merge request pycqa/flake8!436

fix JobsArgument --help output

Closes #665

See merge request pycqa/flake8!434

processor: Catch SyntaxError also when generating tokens for a file

Closes #662

See merge request pycqa/flake8!433

`tokenize.generate_tokens()` can also raise `SyntaxError` in addition to
`tokenize.TokenError`.

treat --extend-exclude as a file list

Closes #653

See merge request pycqa/flake8!432

Normalize paths in CLI-specified config relative to config file's directory

Closes #651

See merge request pycqa/flake8!431

Paths specified in configuration files should be relative to the
directory where the configuration file resides.  Formerly, paths were
normalized relative to the current working directory where `flake8` was
invoked.  The former behavior was not expected, especially for directory
structures with subprojects each having their own configuration.

This sets things up to support normalizing paths relative to parent
directories specified by callers who have more context for determining
what paths should be relative to.