Due to test modules being copied in subdirectories of the
build directory by setuptools, it makes pytest fail by raising
ImportPathMismatchError exceptions when invoked from root
directory of the module.

So ignore the build folder to discover tests.

pytest wastes a lot of time in .hypothesis and .git; this commit excludes them.

pytest-django and pytest-flask both declare a fixture named client and
the wrong one is picked when running swh-deposit tests.

So avoid the loading of pytest flask plugin to prevent errors when
running tests.

We do use the pytest-django plugin to manage the django db setup magic,
so with this change, we can run tests without an existing db, or starting
a postgres db dedicated to the tests using pifpaf.

related to T1261