`testdir` is deprecated. It's identical to `pytester`, except that
`testdir` returns `py.path` objects whereas `pytester` returns Python
stdlib `Path` objects. So I had to update usages of those paths.
`pytester` was added in pytest 6.2, which is our minimum supported
version.
* Fix compatibility with pytest 6 "--import-mode=importlib"
* Rewrite `scenario` and `scenarios` tests so that we can easily parametrize with the different pytest --import-mode.
* Update changelog
* Stop testing for old pytest and python versions
* Switch from pytest-pep8 to pycodestyle.
Pytest-pep8 received the last update in 2014, and it is now not working with pytest >= 4.5
* Explicitly state used markers
* Remove supposedly useless test
* Fix pytest missing markers definitions
* Fix pytest missing markers definitions
* Fix wrong command line usage
* Remove compatibility with ancient pytest
* Be more lenient when checking for failed test string
* Remove dead code
David Röthlisberger