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.
/assign_reviewer @mercurial.review
<!--
Welcome to the Mercurial Merge Request creation process:
* Set a simple title for your MR,
* All important information should be contained in your changesets' content or description,
* You can add some workflow-relevant information here (eg: when this depends on another MR),
* If your changes are not ready for review yet, click `Start the title with Draft:` under the title.
More details here:
* https://www.mercurial-scm.org/wiki/ContributingChanges
* https://www.mercurial-scm.org/wiki/Heptapod
-->