pgsql: ci: macos: Choose python version

ci: macos: Choose python version

The CI base image used to have a python3 with headers etc installed in PATH,
but doesn't anymore. Instead of relying on a specific version in the base
image, explicitly install one ourselves.

On 16 and HEAD this lead to a build without python support, but on 15 CI
failed, due to explicitly enabled python3 support.

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/c6f09e188bbc5df8fd96682ba19e96f19d137eb2

Modified Files
--------------
.cirrus.tasks.yml | 3 +++
1 file changed, 3 insertions(+)