Update GitHub Actions workflows to use main branch instead of master (#1471)

* Update GitHub Actions workflows to use main branch instead of master

All workflow files have been updated to reference the main branch instead of master:
- code_changes.yaml: Updated branch reference and comment
- deploy.yml: Updated branch reference
- pr_code_changes.yaml: Updated branch reference
- pr_docs_changes.yaml: Updated branch reference
- versioning.yaml: Updated branch reference

All workflows already include .github/** in their paths, so they will run on changes to workflow files.

Closes #1470

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Co-Authored-By: Claude <noreply@anthropic.com>

* Run make format

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

---------

Co-authored-by: policyengine-bot <bot@policyengine.org>
Co-authored-by: Claude <noreply@anthropic.com>

Author: 3 people

.github/workflows/code_changes.yaml

@@ -1,10 +1,10 @@
-# Workflow that runs on code changes to the master branch.
+# Workflow that runs on code changes to the main branch.
 
 name: Code changes
 on:
   push:
     branches:
-      - master
+      - main
 
     paths:
       - policyengine_uk/**

.github/workflows/deploy.yml

@@ -5,7 +5,7 @@ name: Deploy documentation
 on:
   push:
     # Runs on pushes targeting the default branch
-    branches: [master]
+    branches: [main]
 env:
   # `BASE_URL` determines, relative to the root of the domain, the URL that your site is served from.
   # E.g., if your site lives at `https://mydomain.org/myproject`, set `BASE_URL=/myproject`.

.github/workflows/pr_code_changes.yaml

@@ -4,7 +4,7 @@ name: Code changes
 on:
   pull_request:
     branches:
-      - master
+      - main
 
     paths:
       - policyengine_uk/**

.github/workflows/pr_docs_changes.yaml

@@ -4,7 +4,7 @@ name: Docs changes
 on:
   pull_request:
     branches:
-      - master
+      - main
 
     paths:
       - docs/**

.github/workflows/versioning.yaml

@@ -1,10 +1,10 @@
 # Workflow that runs on versioning metadata updates.
 
-name: Versioning updates 
+name: Versioning updates
 on:
   push:
     branches:
-      - master
+      - main
 
     paths:
       - changelog_entry.yaml

policyengine_uk/microsimulation.py

@@ -3,7 +3,7 @@
 
 # Third-party imports
 import numpy as np
-from microdf import MicroDataFrame, MicroSeries 
+from microdf import MicroDataFrame, MicroSeries
 
 # PolicyEngine core imports
 from policyengine_core.tracers import SimpleTracer