fix sip4 generator

sip4 does not adhere to PEP-3149
https://peps.python.org/pep-3149/
by default, so we force it to do so in the name of consistency with the sip5 generator.

Author: v4hn

cmake/sip_configure.py

@@ -4,6 +4,7 @@
 import re
 import subprocess
 import sys
+import sysconfig
 
 import sipconfig
 from PyQt5 import QtCore
@@ -43,6 +44,7 @@ def __init__(self):
         macros['INCDIR_QT'] = qtconfig['QT_INSTALL_HEADERS']
         macros['LIBDIR_QT'] = qtconfig['QT_INSTALL_LIBS']
         macros['MOC'] = 'moc-qt5' if find_executable('moc-qt5') else 'moc'
+        macros['EXTENSION_PLUGIN'] = sysconfig.get_config_var('EXT_SUFFIX')[1:] # skip the initial '.' here
         self.set_build_macros(macros)