linkchecker has an odd version check for requests2 that fails on
versions > 2.9.x.
We patch out the check as we are providing a recent requests2. This
patch should be dropped when linkchecker >v9.3 is released.
We now also run the tests although I had to remove one failing test.
Yes, that's covering up the issue but we're still better off than
before.
A few changes from upstream:
1) the executable is patched to support a --no-robots flag to ignore
robots.txto
2) the GUI doesn't work (for now), so this is CLI only