Blender Git Loki

Bump Python to 3.9.7 and add zstandard package

These changes will be committed in two separate consecutive commits.
It's easier to review & build as a whole, hence the single diff.

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*Bump Python 3.9.2 ? 3.9.7*

If we're going to add a Python package to Blender, this usually means
rebuilding all of Python, so we might just as well bump it to the
latest 3.9 release.

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*Bundle the `zstandard` Python package*

This package allows Python scripts to handle compressed blend files.
This is for example needed by #blender_asset_tracer to send files to a
Flamenco render farm.

This change includes a new `WITH_PYTHON_INSTALL_ZSTANDARD` build-time
option, to control whether to actually install the package. For this
the already-existing approach for Requests was copied. I'm not too
happy with the way Python packages and their dependents (Audaspace &
Mantaflow) are handled, but that's for a different cleanup commit.

Differential Revision: https://developer.blender.org/D12777