WIP requirements_common.txt: Upgrade Genshi==0.7.1

[cclauss]

https://pypi.org/project/Genshi is currently failing our build process on Python 3 because of a syntax error. Upgrading to this Python 3 compatible version of Genshi has been attempted before in #1454 but had to be reverted because our Python 2 tests failed on this upgrade.

[tfmorris]

I looked at this as part of the last mega review and my initial impression was that the tests just need to be updated. Genshi is now returning a full HTML doc, instead of just the surrounding HTML snippet.

If the full page is being sanitized in the production code, this should be an issue, but if it's just doing a snippet and then trying to integrate it into the rest of the page, the code may need to get modified. INOW, it may be more than just a test update, but I hope not.

Volunteer needed!

[tfmorris]

The extra html and body tags were a red herring. They were getting added by BeautifulSoup on the error fallback path. The root problem is that the default encoding got changed from utf-8 to None: https://genshi.edgewall.org/wiki/Documentation/upgrade.html#upgrading-from-genshi-0-6-x-to-genshi-0-7-x

Bytes vs strings is a particularly aggravating part of the Python 3 porting and I don't know how much work has been done in this area. The tests here do assert(helpers.sanitize("foo") == "foo") which throws an exception because "foo" represents bytes in Python 3 and there's no encoding give.

Options are:

1. Restore the old default and add encoding = 'utf-8' on the calls.
2. Change the test suite to use use Unicode strings. ie assert(helpers.sanitize(u"foo") == u"foo")

The first is easy and isolated, but may mask problems elsewhere in the system.
The second is a little more work and will get the tests to pass, but may expose problems elsewhere in the system at run time if there are code paths which try to pass bytes instead of strings.

For the second case, we probably also want to change the exception handling so that it doesn't swallow Unicode decoding errors.

Opinions?

[cclauss]

Closed in favor of #1721