Close #19284: Handle -R properly in flag helper

Previously, the -R option would be specified multiple times
if PYTHONHASHSEED was set.
Issue #16129: Py_SetStandardStreamEncoding cleanups

- don't call PyErr_NoMemory with interpreter is not initialised
- note that it's OK to call _PyMem_RawStrDup here
- don't include this in the limited API
- capitalise "IO"
- be explicit that a non-zero return indicates an error
- include versionadded marker in docs
Remove test_sigterm().
Issue #16129: this should appease the buildbots
Remove a duplicate.
Remove redundant empty lines.
Remove redundant empty lines.
Issue #16803: test.test_importlib.test_api now runs under frozen and
source.
Rename Transport.pause/resume to pause_reading/pause_writing. Also relax timeout in test_call_later().
Update pcbuild.sln to build _overlapped.
Issue #18416: Have importlib.machinery.PathFinder treat '' as the cwd
and stop importlib.machinery.FileFinder treating '' as '.'.

Previous PathFinder transformed '' into '.' which led to __file__ for
modules imported from the cwd to always be relative paths. This meant
the values of the attribute were wrong as soon as the cwd changed.
This change now means that as long as the site module is run (which
makes all entries in sys.path absolute) then all values for __file__
will also be absolute unless it's for __main__ when specified by file
path in a relative way (modules imported by runpy will have an
absolute path).

Now that PathFinder is no longer treating '' as '.' it only makes
sense for FileFinder to stop doing so as well. Now no transformation
is performed for the directory given to the __init__ method.

Thanks to Madison May for the initial patch.
merge
Issue #18416: Fix various os calls in importlib.machinery.FileFinder
now that self.path is no longer forced to '.'.
Update more configurations in pcbuild.sln.
fix description of super() behavior on descriptors
merge 3.3
Important race condition fix for Tulip.
Issue #18810: Be optimistic with stat calls when seeing if a directory
exists when checking for a package.

Before there was an isdir check and then various isfile checks for
possible __init__ files when looking for a package.
This change drops the isdir check by leaning
on the assumption that a directory will not contain something named
after the module being imported which is not a directory. If the module
is a package then it saves a stat call. If there is nothing in the
directory with the potential package name it also saves a stat call.
Only if there is something in the directory named the same thing as
the potential package will the number of stat calls increase
(due to more wasteful __init__ checks).

Semantically there is no change as the isdir check moved
down so that namespace packages continue to have no chance of
accidentally collecting non-existent directories.
Add NEWS entry for issue #18810
Fix a refleak in _sre