Tests: Switch to branch 'main' of python-getting-started repo (#1065)

Since the default branch for that repository has been changed from
`master` to `main` in W-7903771.

Closes @W-8047331@.

[skip changelog]

Author: edmorley

hatchet.lock

@@ -1,5 +1,5 @@
 ---
 - - "./repos/python/python-getting-started"
-  - master
+  - main
 - - "./repos/python/python_default"
   - ca947f69027b2a30be5d26f9a42f25e54f4d7a1a