pgsql: Activate Python "Limited API" in PL/Python

Activate Python "Limited API" in PL/Python

This allows building PL/Python against any Python 3.x version and
using another Python 3.x version at run time.  This is useful for
installers that want to run against a separately downloaded Python, so
that they don't have to bundle it themselves.

This builds on the earlier patch to only use APIs supported by the
Limited API.

At the moment, this is not activated on MSVC because that leads to
build failures that no one could explain or cared enough to address.
This could be done later.

Reviewed-by: Jakob Egger <jakob@eggerapps.at>
Discussion: https://www.postgresql.org/message-id/flat/ee410de1-1e0b-4770-b125-eeefd4726a24@eisentraut.org

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/0793ab810038999c0659b3aad9525aa7ef4c8c26

Modified Files
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src/pl/plpython/plpython.h | 9 +++++++++
1 file changed, 9 insertions(+)