Fix CI: use non-deprecated _inferred_freq_str in dt64arr_to_periodarr

The Series.dt.freq deprecation forced the internal caller in
dt64arr_to_periodarr off data.dt.freq, but it landed on
data._values.inferred_freq, which is itself deprecated (GH#64679).
That tripped the Pandas4Warning under -W error in CI. Use the
non-deprecated internal _inferred_freq_str instead, which returns the
identical value the original data.dt.freq did.

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

Author: jbrockmendel

pandas/core/arrays/period.py

@@ -1474,7 +1474,7 @@ def dt64arr_to_periodarr(
         if isinstance(data, ABCIndex):
             data, freq = data._values, data.freq
         elif isinstance(data, ABCSeries):
-            data, freq = data._values, data._values.inferred_freq
+            data, freq = data._values, data._values._inferred_freq_str
 
     elif isinstance(data, (ABCIndex, ABCSeries)):
         data = data._values