Tell pytest not to recurse in dotdirs.

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