Add support for Python 3.11.

Review Request #12498 — Created Aug. 3, 2022 and submitted — Latest diff uploaded


Python 3.11 introduced some internal changes to how functions with
closures and exceptions worked. This led to segfaults when calling our
forwarding functions, as our closure state didn't match what Python
internally expected; and failures representing stack traces, because
internal cached state was never rebuilt.

There's a lot more knowledge internally now in Python 3.11 when it comes
to closures. If a FunctionType and CodeType isn't set up properly,
that internal Python state crashes. The state that matters can't
actually be set by us after defining the function, so it needs to be
properly prepared up-front.

We conditionally do this now by generating the forwarding function code
string in such a way where the forwarding function is inside of another
function. The outer function provides all the variables the inner
function needs in its scope, and the inner forwarding function
references them just enough to satisfy Python.

The generated outer function is then called, returning the inner
forwarding function, which is properly set up for Python 3.11's closer

Exception behavior has also changed. Python 3.11 has improved exception
support, and to facilitate this, the CodeType now keeps track of state
that helps map exceptions to lines, as well as marking ranges of lines
and columns to bytecode. The former is just another argument that goes
into CodeType, but the latter is more internal (accessible, but not
modifiable, by code_type.co_positions()) It seems we can't correctly
set it when setting up the CodeType.

We now use code_type.replace(...) on the forwarding function's
__code__, instead of constructing a CodeType(). This has been
available since Python 3.8, but we've never used it. It's historically
been equivalent to constructing a CodeType() with a function's
defaults, allowing the caller to selectively replace only what's needed.
It now seems to properly rebuild some state needed to ensure the cde
positions all map correctly.

Given the increased complexity in building the forwarding function and
the CodeType, these blocks of logic have been moved into new utility

With these fixes, all unit tests (including a couple new ones) pass on
Python 3.11. This is now listed in our documentation and in our
packaging metadata.

Unit tests pass on Python 2.7 through 3.11.

Performed complete runs of the Review Board and Djblets test suites
using Python 2.7, 3.6, 3.10, and 3.11.