html v2: remove lxml dependency

[bukzor]

The v2 html report shouldn't have any requirements beyond those listed at https://gcc-python-plugin.readthedocs.org/en/latest/basics.html

In terms of python libraries, this seems to consist only of:

• pygments
• six

[bukzor]

WONTFIX

We do some nice things in lxml that would be hard/impossible to do otherwise.