Update mailing list link in developer docs

[brainwane]

We've now decommissioned the pypa-dev Google Group.

I'd like for someone to go through the following developer documentation pages and update all the following links so that they point to https://mail.python.org/mailman3/lists/distutils-sig.python.org/ instead.

This is a good first contribution for someone who's new to this repository. You open up the files listed below and change the URL on the following lines:

• README.rst, line 54:.. _`pypa-dev mailing list`: https://groups.google.com/forum/#!forum/pypa-dev
• docs/development/getting-started.rst, starting on line 26: - For longer-form questions or discussion, message the `pypa-dev mailing
• docs/development/getting-started.rst, line 609: Freenode`_, or the `pypa-dev mailing list`_, to ask questions or get
• docs/development/getting-started.rst, line 633: .. _`pypa-dev mailing list`: https://groups.google.com/forum/#!forum/pypa-dev
• docs/development/index.rst, line 19: or the `pypa-dev mailing list`_, to ask questions or get involved.
• docs/development/index.rst, line 38: .. _`pypa-dev mailing list`: https://groups.google.com/forum/#!forum/pypa-dev

and then open a pull request.

This is a followup to #8010.

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Good First Issue: This issue is good for first time contributors. If you've already contributed to Warehouse, work on another issue without this label instead. If there is not a corresponding pull request for this issue, it is up for grabs. For directions for getting set up, see our Getting Started Guide. If you are working on this issue and have questions, feel free to ask them here, #pypa-dev on Freenode, or the distutils-sig.python.org mailing list.

[Manthan03583]

I want to work on it.

[brainwane]

It looks like @arshadrr didn't see your note before making a pull request, @Manthan03583 -- I've opened #8031 for you to work on instead.

[Manthan03583]

mailing list link in developer docs updated #8030