fix: Don't register berkelydb as a store if it is not available on the system

[eden-logistics]

Summary of changes

Fixes #1816

• Added berkeleydb check in plugin.py
• Removed exception in test_graph.py
• Removed import rdflib.plugins.stores.berkeleydb.has_bsddb

Checklist

• Checked that there aren't other open pull requests for
  the same change.
• Checked that all tests and type checking passes.
• Considered granting push permissions to the PR branch,
  so maintainers can fix minor issues and keep your PR up to date.

[eden-logistics]

Whoops! Sorry about the unprofessional language and weird descriptions in the commit list! I'm still learning how github works and thought it would update differently.

[aucampia]

Whoops! Sorry about the unprofessional language and weird descriptions in the commit list! I'm still learning how github works and thought it would update differently.

No worries :) thanks for the PR, from a quick glance it looks good, will check a bit closer once CI has completed

[aucampia]

Change looks good to me, but maybe the comment at the end of rdflib/plugin.py is not needed.

Thanks again for the PR, I will include some note in the CHANGELOG.md and merge sometime in the next couple of days, probably before friday.

[coveralls]

Coverage increased (+0.2%) to 90.63% when pulling 1058a67 on Jtmcflea:master into a70a9c8 on RDFLib:master.

[aucampia]

I added a CHANGELOG entry to your branch, feel free to change it if you want, I will check it again before I merge it.