Make configure prefer python3 to plain python.

This avoids possibly selecting Python 2.x on systems that have
both Python 2 and Python 3.  We used to feel that what "python"
links to is a user choice that we should honor.  However, we're
about to cease support for Python 2, so users will no longer have
any choice of that sort.  This small change is being made ahead
of the big Python-2-ectomy so that we can see how much of the
buildfarm is not yet prepared for that.  Systems with only
Python 2 will continue to build that way, for now.

Discussion: https://postgr.es/m/2872c9a0-4b0a-1354-d5f6-94d6f85ba354@enterprisedb.com

config/python.m4

@@ -9,14 +9,12 @@
 # Look for Python and set the output variable 'PYTHON' if found,
 # fail otherwise.
 #
-# As the Python 3 transition happens and PEP 394 isn't updated, we
-# need to cater to systems that don't have unversioned "python" by
-# default.  Some systems ship with "python3" by default and perhaps
-# have "python" in an optional package.  Some systems only have
-# "python2" and "python3", in which case it's reasonable to prefer the
-# newer version.
+# Since we are transitioning to supporting only Python 3.x,
+# prefer python3 to plain python.  If the latter exists at all,
+# it very possibly points to python2, which we don't want to
+# select unless it's the only choice.
 AC_DEFUN([PGAC_PATH_PYTHON],
-[PGAC_PATH_PROGS(PYTHON, [python python3 python2])
+[PGAC_PATH_PROGS(PYTHON, [python3 python python2])
 AC_ARG_VAR(PYTHON, [Python program])dnl
 if test x"$PYTHON" = x""; then
   AC_MSG_ERROR([Python not found])

configure

@@ -10279,7 +10279,7 @@ fi
 
 if test "$with_python" = yes; then
   if test -z "$PYTHON"; then
-  for ac_prog in python python3 python2
+  for ac_prog in python3 python python2
 do
   # Extract the first word of "$ac_prog", so it can be a program name with args.
 set dummy $ac_prog; ac_word=$2

doc/src/sgml/installation.sgml

@@ -1875,7 +1875,7 @@ build-postgresql:
          language becomes available.  See
          <xref linkend="plpython-python23"/>
          for more information.  If this is not set, the following are probed
-         in this order: <literal>python python3 python2</literal>.
+         in this order: <literal>python3 python python2</literal>.
         </para>
        </listitem>
       </varlistentry>