Install pandas via pip instead of conda

Conda doesn't have pandas 3.0 in default channel yet, so installing via pip.

Author: mmcky

environment.yml

@@ -4,9 +4,9 @@ channels:
 dependencies:
   - python=3.13
   - anaconda=2025.12
-  - pandas>=3
   - pip
   - pip:
+    - pandas>=3
     - jupyter-book>=1.0.4post1,<2.0
     - quantecon-book-theme==0.15.1
     - sphinx-tojupyter==0.6.0