### NOTE
* We shouldn't merge this PR until GCP cloud functions drops support for Python 3.7 ([Currently scheduled for GCF](https://cloud.google.com/functions/docs/runtime-support#python))
As part of supporting Python 3.12, we're now officially drop support for Python 3.7.
This PR:
* Changed supported Python version from 3.7 to 3.8 in README.
* Replaced distribution test image from `debian:buster` to `debian:bullseye` since the default Python version in buster is 3.7.
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Closes#34450
PiperOrigin-RevId: 622899511
Support Python 3.12.
### Testing
* Passed all Distribution Tests.
* Also tested locally by installing 3.12 artifact.
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### Background
* `distutils` is deprecated with removal planned for Python 3.12
([pep-0632](https://peps.python.org/pep-0632/)), thus we're trying to
replace all distutils usage with setuptools.
* Please note that user still have access to `distutils` if setuptools
is installed and `SETUPTOOLS_USE_DISTUTILS` is set to `local` (The
default in setuptools, more details can be found [in this
discussion](https://github.com/pypa/setuptools/issues/2806#issuecomment-1193336591)).
### How we decide the replacement
* We're following setuptools [Porting from Distutils
guide](https://setuptools.pypa.io/en/latest/deprecated/distutils-legacy.html#porting-from-distutils)
when deciding the replacement.
#### Replacement not mentioned in the guide
* Replaced `distutils.utils.get_platform()` with
`sysconfig.get_platform()`.
* Based on the [answer
here](https://stackoverflow.com/questions/71664875/what-is-the-replacement-for-distutils-util-get-platform),
and also checked the document that `sysconfig.get_platform()` is good
enough for our use cases.
* Replaced `DistutilsOptionError` with `OptionError`.
* `setuptools.error` is exporting it as `OptionError` [in the
code](https://github.com/pypa/setuptools/blob/v59.6.0/setuptools/errors.py).
* Upgrade `setuptools` in `test_packages.sh` and changed the version
ping to `59.6.0` in `build_artifact_python.bat`.
* `distutils.errors.*` is not fully re-exported until `59.0.0` (See
[this issue](https://github.com/pypa/setuptools/issues/2698) for more
details).
### Changes not included in this PR
* We're patching some compiler related functions provided by distutils
in our code
([example](ee4efc31c1/src/python/grpcio/_spawn_patch.py (L30))),
but since `setuptools` doesn't have similar interface (See [this issue
for more details](https://github.com/pypa/setuptools/issues/2806)), we
don't have a clear path to replace them yet.
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* cleanup C# win artifact build
* build C# basictests on win with ninja
* specify default parallelism for C# win artifact
* honor GRPC_PYTHON_BUILD_EXT_COMPILER_JOBS in build_artifact_python.bat
* set --inner_jobs for windows grpc_build_artifact job
* fixup C# build picking env variables from C core build
* get rid of no longer needed NativeDependenciesConfiguration C# setting
* Download the working msys64 and use it to compile 27
* Remove the cygwin detection override logic
* Only install Python interpreters in jobs running on winserver2016
* Fix the batch script substring check
* Increase the timeout for related jobs
* TIL that there is an allow list for Kokoro env vars
Xuan Wang