change URL for bug report

The URL for bug reports now goes to a sagemath.org link
that provides a 301 redirect to whatever is the best
current or future bug reporting platform.
sagenb is not zip safe

This is required for newer versions of setuptools which
will zip up sagenb when installing.
Merge pull request #188 from ohanar/zip_safe

sagenb is not zip safe
Merge pull request #186 from ppurka/issue-redirect-bug-report

change URL for bug report
bump version to 0.10.7.3
Merge pull request #192 from ohanar/version_bump

bump version to 0.10.7.3