devel/py-gitosis: Update to 0.3 and limit to Python 2.7

Python 3 handles octal numbers in a different way as with Python 2.7. The
latter one uses leading zeroes (e.g. 0755) to signify that a number is an
octal number but this is invalid for Python 3 where the required syntax is
0o755 for that case.

Also there are some references to the "cStringIO" module that is gone in
Python 3. Thus limit this port to Python 2.7 only.

While I'm here:

• Add location to license file
• Update URL in pkg-message
• Add test target with the required dependencies
• Pet portlint

Changelog:

https://github.com/tv42/gitosis/releases/tag/release%2F0.3

PR: 235441 [1], 235894 [2]
Submitted by: Keith Gaughan <k@stereochro.me> [2]
Reported by: Dan McGrath <danmcgrath.ca@gmail.com> [1]
Approved by: miwi (mentor), maintainer timeout (> ~25 days) [1], maintainer [2]
Differential Revision: https://reviews.freebsd.org/D19379