Patch based on lines from Georg Brandl, Eric Snow, and Gareth Rees.

Replace it with correct logic that raises ValueError for bad input.
Issues #8478 and #12691 reported the incorrect logic.
Add an Untokenize test case and an initial test method.

Patch by Vajrasky Kok.

Based on patch by Stephen Tu.

This silences a Coverity complain.

to the internal Mac/README file.

on Windows if source name ends with os.altsep.

_active shouldn't be cached, it set to None on shutdown.

This is more RFC compliant (see issue) and fixes a problem with
signature verifiers rejecting the part when signed.  There is some
amount of backward compatibility concern here since it changes
the output, but the RFC issue coupled with fixing the problem
with signature verifiers seems worth the small risk of breaking
code that depends on the current incorrect output.

Patch mostly by Serhiy Storchaka.

I'm checking this in without a test because not much of this code
is tested and I don't have time to work up the necessary extensions
to the existing test framework.

The patch itself was tested by the person who reported the bug.

mmap obmalloc arenas so that they may be immediately returned to the system when unused (closes #20494)

The '**' operator is available only since 8.5 and in any case such large
integers are not supported on Tcl <8.5.

Issue #20426: When passing the re.DEBUG flag, re.compile() displays the debug output every time it is called, regardless of the compilation cache.