[docs] Add Python coding standard to documentation

[thieta]

Summary

As discussed on the forums:

https://discourse.llvm.org/t/rfc-document-and-standardize-python-code-style/

[MatzeB]

llvm/docs/CodingStandards.rst

111–119

We already specify the python version in GettingStarted.rst repeating it here increases the chances that it will get missed and be outdated on the next version bump. So maybe it would be better to not mention any version numbers for python and black here (and possibly link to GettingStarted.rst)?

[jhenderson]

llvm/docs/CodingStandards.rst

111–119

I think it's okay to mention the version number of black here, since it is the only place black will be mentioned in the docs. I agree on the python verison though.

[JDevlieghere]

llvm/docs/CodingStandards.rst

111–119

Agreed with James

117

Is "recommend" strong enough? I was looking for similar wording related to clang-format but it doesn't seem to be part of the coding standards? How about something like:

For consistency and to limit churn, code should be automatically formatted with the black utility.

[thieta]

Hi! After that nudge at EuroLLVM, I picked this one up and hopefully addressed your comments!

[jhenderson]

LGTM, but best wait for others too.

[JDevlieghere]

LGTM

llvm/docs/CodingStandards.rst

 
 Most source code in LLVM and other LLVM projects using these coding standards
 is C++ code. There are some places where C code is used either due to
 environment restrictions, historical restrictions, or due to third-party source
 code imported into the tree. Generally, our preference is for standards
 conforming, modern, and portable C++ code as the implementation language of
 choice.
 
+For automation, build-systems and utility scripts Python is preferred and
+is widely used in the LLVM repository already.
+
 C++ Standard Versions
 ---------------------
 
 Unless otherwise documented, LLVM subprojects are written using standard C++17
 code and avoid unnecessary vendor-specific extensions.
 
 Nevertheless, we restrict ourselves to features which are available in the
 major toolchains supported as host compilers (see :doc:`GettingStarted` page,

 We explicitly avoid some standard facilities, like the I/O streams, and instead
 use LLVM's streams library (raw_ostream_). More detailed information on these
 subjects is available in the :doc:`ProgrammersManual`.
 
 For more information about LLVM's data structures and the tradeoffs they make,
 please consult `that section of the programmer's manual
 <https://llvm.org/docs/ProgrammersManual.html#picking-the-right-data-structure-for-a-task>`_.
 
+Python version and Source Code Formatting
+-----------------------------------------
+
+The current minimum version of Python required is 3.6. Python code in the LLVM
+repository should only use language features available in this version of Python.
+
+The Python code within the LLVM repository should adhere to the formatting guidelines
+outlined in `PEP-8 <https://peps.python.org/pep-0008/>`_.
+
+We recommend automatic code reformatting with the `black <https://github.com/psf/black>`_
+utility. Black allows changing the formatting rules based on major version. In order
+to avoid churn in the formatting rules we currently use black version 23.x in LLVM.
+
 Mechanical Source Issues
 ========================
 
 Source Code Formatting
 ----------------------
 
 Commenting
 ^^^^^^^^^^