(issue #15721)

docstrings and ValueError messages. Patch by Zhongyue Luo

now working with comment block selections. Patch was part of 18226 patch.

Move comment code to a separate function so it can be separately tested.
Original patches by Todd Rovito and Phil Webster.

Before this patch it would raise an error when trying to display
documentation for a method that used annotations.

Patch by Claudiu Popa.

Patch by Antoine Pitrou (based on Robin Schreiber's original patch)

 -I

    Run Python in isolated mode. This also implies -E and -s. In isolated mode
    sys.path contains neither the script’s directory nor the user’s
    site-packages directory. All PYTHON* environment variables are ignored,
    too. Further restrictions may be imposed to prevent the user from
    injecting malicious code.

For 3.3, we just deleted the example.  In 3.4 it was fixed.

Also, document the policy keyword that was added to Message in 3.3.

This was triggered by wanting to make the doctest in email.policy.rst pass;
as_bytes and __bytes__ are clearly useful now that we have BytesGenerator.
Also updated the Message docs to document the policy keyword that was
added in 3.3.