Test types on 3.7 as well

.github/workflows/CI.yml

@@ -162,7 +162,10 @@ jobs:
         shell: bash -l {0}
     strategy:
       matrix:
-        python-version: ['3.11']
+        python-version:
+          - '3.11'
+          - '3.7'
+        os: ['ubuntu-latest']
 
     steps:
       - uses: actions/checkout@v3
@@ -174,8 +177,15 @@ jobs:
       - name: "Install PySR and all dependencies"
         run: |
             python -m pip install --upgrade pip
-            pip install -r requirements.txt
-            pip install mypy jax jaxlib torch
-            python setup.py install
+            python -m pip install -r requirements.txt
+            python -m pip install mypy
+            python -m pip install .
+      - name: "Install additional dependencies"
+        run: python -m pip install jax jaxlib torch
+        if: ${{ matrix.python-version != '3.7' }}
       - name: "Run mypy"
-        run: mypy --install-types --non-interactive pysr
+        run: python -m mypy --install-types --non-interactive pysr
+        if: ${{ matrix.python-version != '3.7' }}
+      - name: "Run compatible mypy"
+        run: python -m mypy --ignore-missing-imports pysr
+        if: ${{ matrix.python-version == '3.7' }}

pysr/export_latex.py

@@ -23,7 +23,7 @@ def sympy2latex(expr, prec=3, full_prec=True, **settings) -> str:
     """Convert sympy expression to LaTeX with custom precision."""
     settings["full_prec"] = full_prec
     printer = PreciseLatexPrinter(settings=settings, prec=prec)
-    return printer.doprint(expr)
+    return str(printer.doprint(expr))
 
 
 def generate_table_environment(