dynamic-import: use sysstr for importing extension and others...
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.
