sys-cluster/charm-6.5.1-r3: Added patch to fix doc.

When building the documentation, we used to have to force <python-3. The added
patch makes this unecessary, which simplifies the ebuild and reduces its
dependencies. The patch was submitted to upstream (Bug 328:
https://charm.cs.illinois.edu/redmine/issues/328).

Package-Manager: portage-2.2.7