Update repo references for rename: drop .myst suffix (#485)

Prepares for renaming lecture-python-programming.myst to
lecture-python-programming. Updates repository_url in _config.yml
and the notebook repo README link.

After merging this PR, rename the repo in GitHub Settings.

Author: mmcky

_notebook_repo/README.md

@@ -4,4 +4,4 @@
 
 Notebooks for https://python-programming.quantecon.org
 
-**Note:** This README should be edited [here](https://github.com/quantecon/lecture-python-programming.myst/.binder)
+**Note:** This README should be edited [here](https://github.com/quantecon/lecture-python-programming/.binder)

lectures/_config.yml

@@ -42,7 +42,7 @@ sphinx:
       dark_logo: quantecon-logo-transparent.png
       header_organisation_url: https://quantecon.org
       header_organisation: QuantEcon
-      repository_url: https://github.com/QuantEcon/lecture-python-programming.myst
+      repository_url: https://github.com/QuantEcon/lecture-python-programming
       nb_repository_url: https://github.com/QuantEcon/lecture-python-programming.notebooks
       path_to_docs: lectures
       twitter: quantecon