Replace use of deprecated Python module distutils.sysconfig, take 2.

With Python 3.10, configure spits out warnings about the module
distutils.sysconfig being deprecated and scheduled for removal in
Python 3.12.  Change the uses in configure to use the module sysconfig
instead.  The logic stays largely the same, although we have to
rely on INCLUDEPY instead of the deprecated get_python_inc function.

Note that sysconfig exists since Python 2.7, so this moves the
minimum required version up from Python 2.6 (or 2.4, before v13).
Also, sysconfig didn't exist in Python 3.1, so the minimum 3.x
version is now 3.2.

Back-patch of commit bd233bdd8 into all supported branches.

In v10, this also includes back-patching v11's beff4bb9c, primarily
because this opinion is clearly out-of-date:

    While at it, get rid of the code's assumption that both the major and
    minor numbers contain exactly one digit.  That will foreseeably be
    broken by Python 3.10 in perhaps four or five years.  That's far enough
    out that we probably don't need to back-patch this.

Peter Eisentraut, Tom Lane, Andres Freund

Discussion: https://postgr.es/m/c74add3c-09c4-a9dd-1a03-a846e5b2fc52@enterprisedb.com

doc/src/sgml/installation.sgml

@@ -193,10 +193,10 @@ su - postgres
       To build the <application>PL/Python</> server programming
       language, you need a <productname>Python</productname>
       installation with the header files and
-      the <application>distutils</application> module.  The minimum
-      required version is <productname>Python</productname> 2.4.
+      the <application>sysconfig</application> module.  The minimum
+      required version is <productname>Python</productname> 2.7.
       <productname>Python 3</productname> is supported if it's
-      version 3.1 or later; but see
+      version 3.2 or later; but see
       <![%standalone-include[the <application>PL/Python</> documentation]]>
       <![%standalone-ignore[<xref linkend="plpython-python23">]]>
       when using Python 3.