testsuite: Fix introspection test on Windows

...when we are using Python 3.8.x or later.  Python 3.8.x or later on Windows
require one to call os.add_dll_directory() on every directory that contains
dependent non-system DLLs of a module that are not bundled/installed with the
module.

Since we are very likely running programs that rely on dependent items in
%PATH%, make things easier for people by calling os.add_dll_directory() on
all the valid paths in %PATH% in api.py, so that the test will run
successfully on Windows with Python 3.8.x or later.

testsuite/introspection/api.py

@@ -1,6 +1,19 @@
 #! /usr/bin/env python3
 
+import os
 import sys
+
+# Python 3.8.x or later on Windows require os.add_dll_directory()
+# to be called on every directory that contains the required
+# non-bundled, non-system DLLs of a module so that the module can
+# be loaded successfully by Python.  Make things easiler for people
+# by calling os.add_dll_directory() on the valid paths in %PATH%.
+if hasattr(os, 'add_dll_directory'):
+    paths = os.environ['PATH'].split(os.pathsep)
+    for path in paths:
+        if path != '' and os.path.isdir(path):
+            os.add_dll_directory(path)
+
 import gi
 
 gi.require_version('Gtk', '4.0')