pgsql: Remove support for Python older than 2.6

Remove support for Python older than 2.6

Supporting very old Python versions is a maintenance burden,
especially with the several variant test files to maintain for Python
<2.6.

Since we have dropped support for older OpenSSL versions in
7b283d0e1d1d79bf1c962d790c94d2a53f3bb38a, RHEL 5 is now effectively
desupported, and that was also the only mainstream operating system
still using Python versions before 2.6, so it's a good time to drop
those as well.

Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://www.postgresql.org/message-id/flat/98b69261-298c-13d2-f34d-836fd9c29b21%402ndquadrant.com

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/37f21ed132d1c5aee88e81fee0a0b7e735673d35

Modified Files
--------------
config/python.m4                                   |   4 +-
configure                                          |   4 +-
doc/src/sgml/installation.sgml                     |   2 +-
doc/src/sgml/plpython.sgml                         |  16 +-
src/pl/plpython/expected/README                    |   4 -
src/pl/plpython/expected/plpython_error_0.out      | 447 --------------------
.../plpython/expected/plpython_subtransaction.out  |  65 ---
.../expected/plpython_subtransaction_0.out         | 448 ---------------------
.../expected/plpython_subtransaction_5.out         | 448 ---------------------
src/pl/plpython/plpy_elog.c                        |   6 -
src/pl/plpython/plpython.h                         |  29 --
src/pl/plpython/sql/plpython_subtransaction.sql    |  37 --
12 files changed, 7 insertions(+), 1503 deletions(-)