Upgrade pytest-mozwebqa plugin

[davehunt]

This pull upgrades to pytest-mozwebqa 0.8, and also updates py/pytest/pytest-xdist dependencies.

After merging, we should do the following to any relevant Jenkins jobs:

• Point to the new location of the requirements file
• Use seconds instead of milliseconds for the timeout command line option
• Use -m instead of -k for specifying mark filters
• Enabling archiving of HTML reports (as these are now much smaller than before)

[AlinT]

Lgtm.