MINOR: [Python] Document Python 3.9 support
MINOR: [Python] Document Python 3.9 support

Missed in #8386.

Closes #10286 from adamchainz/patch-1

Authored-by: Adam Johnson <b1c1d8736f20db3fb6c1c66bb1455ed43909f0d8@adamj.eu>
Signed-off-by: David Li <dc24f9cfc84fb1675ce65c8444a2c3e2f0e5f297@gmail.com>

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