Fix pandas DeprecationWarning for is_period_dtype in anoph/frq_base.py #1366

Running poetry run pytest tests/anoph/test_snp_frq.py with pandas 2.2.3 (Python 3.12, macOS) emits a DeprecationWarning from malariagen_data/anoph/frq_base.py at lines 73 and 118:
is_period_dtype is deprecated and will be removed in a future version. Use isinstance(dtype, pd.PeriodDtype) instead

pandas deprecated this function in 2.2.0. The tests pass, but the call will break when pandas removes it.

Proposed fix: replace both calls with isinstance(dtype, pd.PeriodDtype). I’ve made this change locally and the tests pass with the warning gone. Happy to open a PR if that works for you.