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.

BasedOnStyle: LLVM
IndentWidth: 8
UseTab: ForIndentation
BreakBeforeBraces: Linux
AllowShortIfStatementsOnASingleLine: false
IndentCaseLabels: false
AllowShortBlocksOnASingleLine: false
AllowShortFunctionsOnASingleLine: false
IncludeCategories:
  - Regex:           '^<'
    Priority:        1
  - Regex:           '^"'
    Priority:        2