Simplified get_user_id() and fixed possible python3 compatiblity issue.

Changed motivated by https://github.com/gitpython-developers/GitPython/pull/52
Fixed premature closing of stdout/stderr streams, which caused plenty of errors.

The lines were added in commit b38020ae , and I might consider a patch release soon
or get ready with 0.3.3. Lets hope not too many installations will be affected.
Don't use tuples in setup.py requirement specs

[skip ci]
See https://github.com/gitpython-developers/GitPython/issues/186 for
the motivation of this fix.