dynamic-import: use sysstr for importing extension and others
This logic is used by extensions, and python hooks and merge-tools. All this
logic eventually deals with native string (unicode in Python 3). This patch
makes it handle `str` directly instead of relying on some pycompat low lever
layer to do the conversion at the last minutes.
We adjust the Python version filtering of a test as the output seems to be present with Python 3.7 too.
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273ce12ad8f1` will report some style violations on a very old commit).