Replace treading.local with contextvar

[dokime7]

In order to use 'transaction' on an asyncio framework like FastAPI, it's needed to use ContextVar instead of threading.local that is not compatible with asyncio.

ContextVar is fully compatible with threading.local and can be used as a replacement of.

ContextVar is a Python standard lib since 3.7 and can be installed as a lib on 3.6 but unfortunately not compatible with older Python versions.

[jugmac00]

@dokime7 Thanks for the PR - please note, that this package needs to stay compatible with Python 2.7 and Python 3.5

[dokime7]

OK, I will update my PR

[dokime7]

Updated, feel free to give feedback.

[icemac]

Thank you for your contribution.

According to the contributing policies of the zopefoundation organization you need to sign a contributor agreement before any non-trivial change can be merged. For details please consult the Contributing guidelines for zopefoundation projects.

[icemac]

This package uses https://pypi.org/project/coverage-python-version/ so marking the code paths needed for Python 2 resp. 3 should bring the coverage back to the expected value.

[dokime7]

OK, I've pushed a fix

[icemac]

That's what I meant with my comment regarding https://pypi.org/project/coverage-python-version/.

Explanation: We are running coverage only on Python 3.8 so code needed just for older Python versions (especially) Python 2.7 should be marked with # pragma: PY2 instead of no cover as we want to see coverage for code which has different branches for different Python versions.

[dokime7]

OK, sorry, it is much clear for me now :)

Requested changes where applied.

[dataflake]

This still requires signing the contributor agreement. Please see the Contributing guidelines for zopefoundation projects.

[gforcada]

If you switch the if order it might make the diff much more easy to read:

if not USE_CONTEXTVAR:
   # old/current code that on the diff is only shown as being indented
else:
  # new code, that shows up as new code

otherwise a quick glimse at the code feels like the new code is actually the old one 😅