Pull in xrootd-python as dependency

The setup.py pulls in the patched xrootd-python explicitly:

git.Git("").clone("https://github.com/KIT-ETP-Computing/xrootd-python.git")

This can be solved by listing xrootd-python in the install_requires and the repository in the requirements.txt or dependency_links. Both pip and setup.py install can use this.

• remove gitpython as a dependency, and the entire fetching/building/installing code
• add xrootd-python as a dependency
• add git+https://github.com/KIT-ETP-Computing/xrootd-python.git@master#egg=xrootd-python-0.3.0 to the dependency_links

See also: Stackoverflow: pip ignores dependency_links in setup.py