Add a GHA to enforce version bump

.github/workflows/check_fits_version.yaml

@@ -0,0 +1,54 @@
+name: Check that the version number was bumped if _fits.py was modified
+
+on:
+  pull_request:
+
+jobs:
+  check_version:
+    runs-on: ubuntu-latest
+
+    steps:
+    - name: Checkout code
+      uses: actions/checkout@v3
+      with:
+        fetch-depth: 0
+
+    - name: Check if _fits.py has changed
+      id: check_file_changes
+      run: |
+        # 'github.event.before/after' and 'github.event.pull_request.base.sha' are inconsistent for PRs
+        # Find the commit where the PR branch was forked from the base branch
+        git fetch origin ${{ github.base_ref }} ${{ github.event.pull_request.head.ref }}
+        FORK_POINT=$( git show-branch --merge-base "origin/${{ github.base_ref }}"\
+                      "origin/${{ github.event.pull_request.head.ref }}" )
+        LATEST_COMMIT_IN_PR=${{ github.event.pull_request.head.sha }}
+        CHANGED_FILES=$( git diff --name-only $FORK_POINT $LATEST_COMMIT_IN_PR )
+        if [[ $CHANGED_FILES == *"python/lsst/cell_coadds/_fits.py"* ]]; then
+          echo "FILE_CHANGED='true'" >> $GITHUB_ENV
+        else
+          echo "FILE_CHANGED='false'" >> $GITHUB_ENV
+        fi
+
+    - name: Get previous version
+      id: get_previous_version
+      if: ${{ env.FILE_CHANGED }} == 'true'
+      run: |
+        PREVIOUS_VERSION=$(git show HEAD^:python/lsst/cell_coadds/_fits.py |\
+                           grep 'MODULE_VERSION =' | cut -d'=' -f2 | tr -d '[:space:]')
+        echo "PREVIOUS_VERSION=$PREVIOUS_VERSION" >> $GITHUB_ENV
+
+    - name: Get current version
+      id: get_current_version
+      if: ${{ env.FILE_CHANGED }} == 'true'
+      run: |
+        CURRENT_VERSION=$(grep 'MODULE_VERSION = ' python/lsst/cell_coadds/_fits.py | cut -d'=' -f2 | tr -d '[:space:]')
+        echo "CURRENT_VERSION=$CURRENT_VERSION" >> $GITHUB_ENV
+
+    - name: Compare versions
+      id: compare_versions
+      if: ${{ env.FILE_CHANGED }} == 'true'
+      run: |
+        if [[ "${{ env.PREVIOUS_VERSION }}" == "${{ env.CURRENT_VERSION }}" ]]; then
+          echo "Error: _fits.py was modified but MODULE_VERSION was not!"
+          exit 1
+        fi

python/lsst/cell_coadds/_fits.py

@@ -26,7 +26,7 @@
 to know which files written by an earlier version of this module is still
 readable or not. Any changes to this module, even as trivial as formatting
 changes and documentation updates should bump the version number (in this case,
-it would bump the patch).
+it would bump the patch). This is mandated in GitHub Actions.
 """
 
 from __future__ import annotations