Exclude Python 3.9 runs for milestone branch. (ansible-collections#1700)

 GHA: Exclude Python 3.9 runs for milestone branch

SUMMARY
At least for sanity tests.
Ref: ansible-collections/news-for-maintainers#56
ISSUE TYPE

Test Pull Request

COMPONENT NAME
GHA sanity tests

Reviewed-by: Alina Buzachis
Reviewed-by: Mark Chappell

.github/workflows/sanity.yml

@@ -53,6 +53,10 @@ jobs:
               "ansible-version": "milestone",
               "python-version": "3.8"
             },
+            {
+              "ansible-version": "milestone",
+              "python-version": "3.9"
+            },
             {
               "ansible-version": "devel",
               "python-version": "3.7"