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_10_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/cd1873160d9b9b7002814efeff28bb5c21daadce

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