ARROW-10340: [Packaging][deb][RPM] Use Python 3.8 for pygit2
ARROW-10340: [Packaging][deb][RPM] Use Python 3.8 for pygit2

Because pygit2 wheel for Python 3.9 isn't available yet.

Closes #8488 from kou/packaging-linux-use-python-3.8

Authored-by: Sutou Kouhei <73218711cdd5d5c30ac66da3cc4d9d9204caf34b@clear-code.com>
Signed-off-by: Sutou Kouhei <73218711cdd5d5c30ac66da3cc4d9d9204caf34b@clear-code.com>

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