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.

Before:

$ time pytest --collect-only > /dev/null
pytest --collect-only > /dev/null  6.93s user 0.51s system 100% cpu 7.425 total

After:

$ time pytest --collect-only > /dev/null
pytest --collect-only > /dev/null  2.39s user 0.10s system 100% cpu 2.475 total