Pin VSCode Python extension to avoid Jedi snippets

The VSCode Python extension altered the behavior of snippet expansion
when using the open-source Jedi LSP, adding parentheses after class
names. This behavior may be bothersome when adding type annotations.
See: microsoft/vscode-python#15858

The original issue report lists `ms-python.python@2021.3.680753044` as
the problematic version, but for me, this behavior began later, with
`ms-python.python@2021.10.1317843341`. Installing a prior release,
`ms-python.python@2021.9.1246542782`, avoids this behavior.

The VSCode extension marketplace only displays the last five versions of
each extension, but a longer version history is available from Open VSX.
See: https://open-vsx.org/extension/ms-python/python/2021.9.1246542782

The VSCode CLI can also apparently install specific extension versions:
`code --install-extension ms-python.python@2021.9.1246542782`.
See: microsoft/vscode#136313

vscode/extensions/marketplace-open-vsx.txt

@@ -7,7 +7,7 @@ esbenp.prettier-vscode
 foxundermoon.shell-format
 hashicorp.terraform
 mikestead.dotenv
-ms-python.python
+ms-python.python@2021.9.1246542782
 octref.vetur
 redhat.vscode-yaml
 sourcery.sourcery