This commit adds a new runner mode to stestr that enables users to run
test suites using pytest instead of unittest. This is an opt-in feature
as for compatibility reasons we'll always default to using unittest, as
all existing stestr users have only been leveraging unittest to run
tests. A pytest plugin that adds a subunit output mode is now bundled
with stestr. When the user specifies running tests with pytest it calls
out to pytest just as stestr does with the integrated unittest extension
today and sets the appropriate flags to enable subunit output.

To facilitate this new feature pytest is added to the stestr
requirements list. I debated making it an optional dependency, but to
make it easier for the significantly larger pytest user base (it's
downloaded ~900x more per month) it seemed simpler to just make it a
hard requirement. But I'm still not 100% on this decision so if there
is sufficient pushback we can start it out as an optional dependency.

Co-Authored-By: Joe Gordon <[email protected]>

Closes: #354

This commit adds a return code subclass that executes the tests in
pytest mode written with the pytest test suite. These tests caught a
couple of issues when running in pytest mode.

This commit adds a new CI job that runs stestr's unit test suite in
pytest mode instead of using stestr's built in subunit runner (based on
stdlib unittest).

This commit fixes the mocking used for the config file tests. It was
previously implicitly defining a runner argument as a mock object which
wasn't a valid choice for a config file to contain after parsing. This
commit just explicitly sets the mock to have the unittest runner set to
avoid failure.

…n windows"

This reverts commit cbbe2fc.

The pytest subunit plugin was loading test paths incorrectly on windows
as a posix path. This was causing the test loading to fail unexpectedly
because pytest was unable to process the posix path version of a windows
path. This commit fixes this issue in the pytest plugin.