MNT: revert unrelated requirements.txt version pins

Keep the PR scoped to the animation feature; requests/pytz pinning was
unrelated.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: Gui-FernandesBR

requirements.txt

@@ -2,7 +2,7 @@ numpy>=1.13
 scipy>=1.0
 matplotlib>=3.9.0  # Released May 15th 2024
 netCDF4>=1.6.4
-requests>=2.25.0
-pytz>=2020.1
+requests
+pytz
 simplekml
 dill