Create workflow to update the backend automatically

.github/workflows/update_backend.yml

@@ -0,0 +1,58 @@
+name: PySR backend update
+on:
+  schedule:
+    - cron: '00 00 * * *'
+  workflow_dispatch:
+jobs:
+  update_compat:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v3
+      - uses: julia-actions/setup-julia@v1
+      - uses: julia-actions/cache@v1
+      - uses: actions/setup-python@v4
+
+      - name: "Install PySR"
+        run: |
+          python -m pip install --upgrade pip
+          pip install -r requirements.txt
+          python setup.py install
+          # Not needed:
+          # python -c 'import pysr; pysr.install()'
+
+      - name: "Get SymbolicRegression.jl latest version"
+        id: get-latest
+        run: |
+          cd $(mktemp -d)
+          git clone https://github.com/MilesCranmer/SymbolicRegression.jl
+          cd SymbolicRegression.jl
+          echo "version=$(git describe --tags --abbrev=0 | sed 's/^v//')" >> $GITHUB_OUTPUT
+
+      - name: "Get SymbolicRegression.jl version used in PySR"
+        id: get-current
+        run: |
+          echo "version=$(python -c 'import pysr; print(pysr.version.__symbolic_regression_jl_version__)' 2>/dev/null)" >> $GITHUB_OUTPUT
+
+      # If versions are different, we want to take our checked-out version,
+      # create a new branch called "update_compat_{...}", where the "..."
+      # is a timestamp. We then want to
+      # go to pysr/version.py, bump the patch version of PySR (__version__),
+      # set the version of __symbolic_regression_jl_version__ to the latest
+      # version of SymbolicRegression.jl, and then commit and push.
+      # Finally, we will open a PR from this branch to master.
+      - name: "Update versions"
+        if: ${{ steps.get-latest.outputs.version != steps.get-current.outputs.version }}
+        run: |
+          # Bump PySR patch number:
+          CURRENT_PYSR_PATCH_VERSION=$(python -c 'import pysr; print(pysr.version.__version__.split(".")[-1])' 2>/dev/null)
+          NEW_PYSR_PATCH_VERSION=$((CURRENT_PYSR_PATCH_VERSION + 1))
+          sed -i "s/^__version__ = .*/__version__ = \"$(python -c 'import pysr; print(".".join(pysr.version.__version__.split(".")[:-1]))' 2>/dev/null).${NEW_PYSR_PATCH_VERSION}\"/" pysr/version.py
+          
+          # Set SymbolicRegression.jl version:
+          sed -i "s/^__symbolic_regression_jl_version__ = .*/__symbolic_regression_jl_version__ = \"${{ steps.get-latest.outputs.version }}\"/" pysr/version.py
+
+          git add pysr/version.py
+          git commit -m "Update backend version to v${{ steps.get-latest.outputs.version }}"
+
+      - name: "Create PR"
+        uses: peter-evans/create-pull-request@v3