bpo-41304: Ensure python3x._pth is loaded on Windows (GH-21495)

(cherry picked from commit 936a66094591dc0e67d4a60c170148bb700ec016)

Co-authored-by: Steve Dower <steve.dower@python.org>
diff --git a/PC/getpathp.c b/PC/getpathp.c
index 0939c5f..53da3a6 100644
--- a/PC/getpathp.c
+++ b/PC/getpathp.c
@@ -669,7 +669,7 @@
 get_pth_filename(PyCalculatePath *calculate, wchar_t *filename,
                  const _PyPathConfig *pathconfig)
 {
-    if (get_dllpath(filename) &&
+    if (!get_dllpath(filename) &&
         !change_ext(filename, filename, L"._pth") &&
         exists(filename))
     {