Flake8 now ignores testdata
build: fix `python_finder`: prefer python2, look for/prefer .so

This fixes building YCM / ycmd for pyenv with python3 and python2
activated/configured.
Make check_third_party_libs only look into folders

I had a `.rej` file from `patch` for a submodule, and
check_third_party_libs complained about it.
Merge branch 'fix-build-python-config'

Conflicts:
	build.sh
Use persistent build dir with 'install'