Update pytest-cov to 4.0.0

[pyup-bot]

This PR updates pytest-cov from 3.0.0 to 4.0.0.

Changelog

4.0.0

------------------

**Note that this release drops support for multiprocessing.**


* `--cov-fail-under` no longer causes `pytest --collect-only` to fail
Contributed by Zac Hatfield-Dodds in `511 <https://github.com/pytest-dev/pytest-cov/pull/511>`_.
* Dropped support for multiprocessing (mostly because `issue 82408 <https://github.com/python/cpython/issues/82408>`_). This feature was
mostly working but very broken in certain scenarios and made the test suite very flaky and slow.

There is builtin multiprocessing support in coverage and you can migrate to that. All you need is this in your
``.coveragerc``::

 [run]
 concurrency = multiprocessing
 parallel = true
 sigterm = true
* Fixed deprecation in ``setup.py`` by trying to import setuptools before distutils.
Contributed by Ben Greiner in `545 <https://github.com/pytest-dev/pytest-cov/pull/545>`_.
* Removed undesirable new lines that were displayed while reporting was disabled.
Contributed by Delgan in `540 <https://github.com/pytest-dev/pytest-cov/pull/540>`_.
* Documentation fixes.
Contributed by Andre Brisco in `543 <https://github.com/pytest-dev/pytest-cov/pull/543>`_
and Colin O'Dell in `525 <https://github.com/pytest-dev/pytest-cov/pull/525>`_.
* Added support for LCOV output format via `--cov-report=lcov`. Only works with coverage 6.3+.
Contributed by Christian Fetzer in `536 <https://github.com/pytest-dev/pytest-cov/issues/536>`_.
* Modernized pytest hook implementation.
Contributed by Bruno Oliveira in `549 <https://github.com/pytest-dev/pytest-cov/pull/549>`_
and Ronny Pfannschmidt in `550 <https://github.com/pytest-dev/pytest-cov/pull/550>`_.

Links

• PyPI: https://pypi.org/project/pytest-cov
• Changelog: https://pyup.io/changelogs/pytest-cov/
• Repo: https://github.com/pytest-dev/pytest-cov