Python 2.2 is no longer supported

It was already on its last legs, and it turns out that it was
accidentally broken in commit 89e850e6
and no one cared.  So remove the rest the support for it and update
the documentation to indicate that Python 2.3 is now required.

doc/src/sgml/installation.sgml

@@ -206,7 +206,7 @@ su - postgres
       installation with the header files and
       the <application>distutils</application> module.  The minimum
       required version is <productname>Python</productname>
-      2.2.  <productname>Python 3</productname> is supported if it's
+      2.3.  <productname>Python 3</productname> is supported if it's
       version 3.1 or later; but see
       <![%standalone-include[the <application>PL/Python</> documentation]]>
       <![%standalone-ignore[<xref linkend="plpython-python23">]]>

src/pl/plpython/expected/README

@@ -9,6 +9,3 @@ plpython_subtransaction_0.out	Python 2.4 and older (without with statement)
 plpython_subtransaction_5.out	Python 2.5 (without with statement)
 
 plpython_types_3.out			Python 3.x
-
-Note: Building with Python 2.2 is supported, but there are no expected
-files for it (too much work to maintain).

src/pl/plpython/plpython.h

@@ -67,13 +67,6 @@ typedef int Py_ssize_t;
 #define PY_SSIZE_T_MIN INT_MIN
 #endif
 
-/*
- * PyBool_FromLong is supported from 2.3.
- */
-#if PY_VERSION_HEX < 0x02030000
-#define PyBool_FromLong(x) PyInt_FromLong(x)
-#endif
-
 /*
  * Python 2/3 strings/unicode/bytes handling.  Python 2 has strings
  * and unicode, Python 3 has strings, which are unicode on the C