Similarly to wendelin.core (see ad34ff4a "wendelin.core: Always use git
checkout for both release and development version") let's switch pygolang to
install via git checkout for both release and tests.

This way we avoid dependency on pypi and will be able to do minor updates
without doing bigger to-pypi releases. Always using git checkout also helps
security a bit because we remove reliance on trust to tarball received from pypi.

/cc @jerome, @kazuhiko