Fix deprecation warnings for pandas, workaround freq change, pin ruff up

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 78.26087% with 5 lines in your changes are missing coverage. Please review.

Project coverage is 57.32%. Comparing base (56eb7dd) to head (a492aa1).

Files	Patch %	Lines
ffn/core.py	78.26%	2 Missing and 3 partials :warning:

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #224 +/- ## ========================================== + Coverage 57.15% 57.32% +0.17% ========================================== Files 4 4 Lines 1160 1174 +14 Branches 249 254 +5 ========================================== + Hits 663 673 +10 - Misses 433 435 +2 - Partials 64 66 +2 ```

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