pgsql: configure: Update python search order

configure: Update python search order

Some systems don't ship with "python" by default anymore, only
"python3" or "python2" or some combination, so include those in the
configure search.

Discussion: https://www.postgresql.org/message-id/flat/1457.1543184081%40sss.pgh.pa.us#c9cc1199338fd6a257589c6dcea6cf8d

Branch
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REL_11_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/3d498c65ac6ed69aa6ede68186420b3c0dadf3c7

Modified Files
--------------
config/python.m4               | 9 ++++++++-
configure                      | 2 +-
doc/src/sgml/installation.sgml | 3 ++-
3 files changed, 11 insertions(+), 3 deletions(-)