feat: Allow 100 Dependabot PRs for main Python dependencies

In case there's another case of a dependency blocking everything, such as
<aws/aws-cdk#21867>.

(Not setting to 0 (unlimited) in case of any future bugs in Dependabot
creating crazy amounts of PRs.)

.github/dependabot.yml

@@ -20,6 +20,7 @@ updates:
       interval: daily
   - package-ecosystem: pip
     directory: /
+    open-pull-requests-limit: 100
     schedule:
       interval: daily
   - package-ecosystem: pip