conftest: use pytest_ignore_collect collection_path to address pytest deprecation warning

It said:
PytestRemovedIn9Warning: The (path: py.path.local) argument is deprecated, please use (collection_path: pathlib.Path)
see https://docs.pytest.org/en/latest/deprecations.html#py-path-local-arguments-for-hooks-replaced-with-pathlib-path
def pytest_ignore_collect(path):

conftest: Refactor (simplify and optimize) pytest_ignore_collect

Move constants out of the function, and return None explicitly. No big deal, but better.

py3: work around incompatibility between pytest, py3 inspect, and tg

Work around an issue that has been reported on
https://github.com/TurboGears/tg2/issues/118 :

.../site-packages/_pytest/doctest.py:381: in _mock_aware_unwrap
return real_unwrap(obj, stop=_is_mocked)
/usr/lib64/python3.7/inspect.py:511: in unwrap
while _is_wrapper(func):
/usr/lib64/python3.7/inspect.py:505: in _is_wrapper
return hasattr(f, '__wrapped__') and not stop(f)
.../site-packages/tg/support/objectproxy.py:19: in __getattr__
return getattr(self._current_obj(), attr)
.../site-packages/tg/request_local.py:240: in _current_obj
return getattr(context, self.name)
.../site-packages/tg/support/objectproxy.py:19: in __getattr__
return getattr(self._current_obj(), attr)
.../site-packages/tg/support/registry.py:72: in _current_obj
'thread' % self.____name__)
E TypeError: No object (name: context) has been registered for this thread

pytest's doctest support is (in _mock_aware_unwrap) using py3 inspect.

Inside inspect, _is_wrapper will do an innocent looking:
hasattr(f, '__wrapped__')

But if the code under test has un (unused) import of a tg context (such as
tg.request), it is no longer so innocent. tg will throw:
TypeError: No object (name: context) has been registered for this thread
(which in py2 would have caught by hasattr, but not in py3.)

pytest will thus fail already in the "collecting ..." phase.

To work around that, use the hack of pushing a tg context in the top level
pytest_configure.

pytest: fix register_assert_rewrite call

Since a while, the test suite shows following warning:

kallithea/tests/__init__.py:29
/home/tdescham/repo/contrib/kallithea/kallithea-review/kallithea/tests/__init__.py:29: PytestAssertRewriteWarning: Module already imported so cannot be rewritten: kallithea.tests
pytest.register_assert_rewrite('kallithea.tests')

The problem can be fixed by moving the register_assert_rewrite call from
kallithea/tests/__init__.py to the root-level conftest.py, outside of the
'kallithea' module.

tests: introduce doctest_mock_ugettext to allow doctests of localized code

Future doctests will require some extra mocking, as the code-under-test uses
translation (ugettext aka '_') and its provider TurboGears2 needs a context.

Avoid this complexity by mocking ugettext as the identity function. This is
done by providing a pytest fixture 'doctest_mock_ugettext' that will mock
uggettext in the module that uses the fixture.

tests: run doctests via pytest

pytest can run doctests as part of the standard testsuite run. See
documentation at [1].

pytest will discover doctests in all python files it can find. However, some
files cannot be imported directly in this manner.
Fix this by adding a filter in conftest.py in the root directory. This code
could also live in kallithea/conftest.py, but it cannot be in
kallithea/tests/conftest.py because that level is deeper than the files we
want to filter.

[1] https://docs.pytest.org/en/latest/doctest.html