Add support for Python 3.8

modified:   .travis.yml
modified:   tox.ini

Fix: handle verbose include/exclude patterns in pyproject.toml

If a verbose pattern was being used in pyproject.toml, the
patterns did not function correctly. For instance, the pattern

exclude = '''
(
/(
  folder1
| folder2
)/
)
'''

does not function properly. This pattern is inspired by the
pattern that black themselves use:
https://github.com/python/black/blob/master/pyproject.toml

Solve this issue by removing newlines, and formatting
the pattern before using it. This is done the same way
as black themselves are doing it.

Remove black version number to fix #38

Merge pull request #43 from jonasundderwolf/pytest-deprecation-fix

Remove deprecation warning at startup

Remove upperbound for required black version

As long as there is no reason to restrict the black version I think it should not be pinned

Allow pytest-black to be installed on and 'enabled' on Python 2.7+. (#34

)

fix: address PytestUnknownMarkWarning

As of pytest 4.5.0, a new warning is raised when the plugin doesn't
register its markers. See pytest-dev/pytest#5177

The docs recommend registering custom markers:
https://docs.pytest.org/en/latest/writing_plugins.html#registering-markers

Signed-off-by: Mike Fiedler <miketheman@gmail.com>

Upgrade black version to latest (19.3b0)

black==19.3b0 was released on 2019-03-14

See https://pypi.org/project/black/#history

Use setuptools_scm to extract version num from git

Bump version to 0.3.3 (#24)