[stable/yoga] Fix pep8 error in assert type comparison

[rodrigogansobarbieri]

charmhelpers/fetch/snap.py:55:12: E721 do not compare types, for exact checks use is / is not, for instance checks use isinstance()

(cherry picked from commit 5b0a3e6aa3e3141d70120aad7dd2c321595d7225) (cherry picked from commit 452988fad4a5833d6946fd92906c57bee0b37fe7)

[ajkavanagh]

Merged on a single approval on stable branch as it is, in effect, a no-op to keep pep8 happy.