Handle import of optional dependencies consistently

[simonbohnen]

@xhochy suggested this pattern for optional dependencies like gcsfs:

minimalkv/minimalkv/net/gcstore.py

Lines 7 to 12 in cff85c0

	try:
	from gcsfs import GCSFileSystem
	has_gcsfs = True
	except ImportError:
	has_gcsfs = False

We should apply this to all the dependencies, e.g. dulwich:

minimalkv/minimalkv/git.py

Lines 6 to 7 in cff85c0

	from dulwich.objects import Blob, Commit, Tree
	from dulwich.repo import Repo