Support Python 3.9

[icemac]

Look through the change log of Python 3.9 for potential issues which need to be handled by RestrictedPython to prevent access to otherwise forbidden data.

[dataflake]

I have scanned the Python 3.9-related parts of https://docs.python.org/3.9/whatsnew/changelog.html and couldn't see anything obvious. I paid special attention to the modules that are exposed as "safe", simple types and ast.

[icemac]

Thank you. We should wait for the final release until we officially declare that we support it.

Note to self: Python 3.9b4 was the released version at this time.

[dataflake]

https://www.python.org/dev/peps/pep-0596/ mentions Monday October 5th as 3.9.0 release day

[icemac]

Reviewed change log up to 3.9.0rc2 which is planned as the last non-final release before 3.9-final.

So only reviewing the changes in the final release has to be done.

[icemac]

Additionally the now operations | and |= on dictionaries have to be checked to work along the security restrictions.

[d-maurer]

Michael Howitz wrote at 2020-10-5 23:09 -0700:

Additionally the operators `|` and `|=` on dictionaries have to be checked to work along the security restrictions.

"zopefoundation/AccessControl#98" gives examples that already for Python 3.8 (and before) some "special methods" could be used indirectly by untrusted code.

[icemac]

There is nothing problematic between Python 3.9.0rc2 and 3.9.0 final in the change log.

[icemac]

• RestrictedPython transcribes a |= {'b': 1} to _inplacevar_("|=", a, {'b': 1}), so it depends on the implementation of _inplacevar_ what happens with such code using the new augmented assign for dicts in Python 3.9. This implementation has to be provided by the code using RestrictedPython.
  Zope prevents using |= for dicts, see https://github.com/zopefoundation/AccessControl/blob/f8c05c03556da188ec8331fc281f3595e9170b57/src/AccessControl/ZopeGuards.py#L648 (dict is not in valid_inplace_types.)
• | does not change anything in place but creates a new dict, so it is harmless.

Conclusion: The new bit-or operators in Python 3.9 do not require changes in RestrictedPython.