Pin sympy to 1.13.3 to avoid 1.14.0 bug

- SymPy 1.14.0 (bundled with Anaconda 2025.12) has a regression causing
  OverflowError in solve() for symbolic differentiation
- Bug occurs in solow.md when solving c.diff() due to factorint issue
  with large mpz integers trying to convert to float
- Pinning to 1.13.3 which works correctly with Python 3.13
- Added TODO to check for SymPy 1.14.1+ when available

Author: mmcky

environment.yml

@@ -15,5 +15,9 @@ dependencies:
     - sphinxcontrib-youtube==1.4.1
     - sphinx-togglebutton==0.3.2
     - sphinx-reredirects==1.0.0
+    # TODO: Check for SymPy 1.14.1+ when available
+    # Pinned to 1.13.3 due to bug in 1.14.0 causing OverflowError in solve() 
+    # for symbolic differentiation in solow.md (factorint issue with large mpz)
+    - sympy==1.13.3