remove is_period and change str representation for freq in Period [skip ci]

Author: natmokval

Diff for: pandas/_libs/tslibs/period.pyx

@@ -2372,7 +2372,7 @@ cdef class _Period(PeriodMixin):
         """
         Return a string representation of the frequency.
         """
-        return self.freq.freqstr
+        return self.freq.freqstr.replace("ME", "M")
 
     def __repr__(self) -> str:
         base = self._dtype._dtype_code

Diff for: pandas/conftest.py

@@ -919,7 +919,7 @@ def rand_series_with_duplicate_datetimeindex() -> Series:
     params=[
         (Interval(left=0, right=5), IntervalDtype("int64", "right")),
         (Interval(left=0.1, right=0.5), IntervalDtype("float64", "right")),
-        (Period("2012-01", freq="ME"), "period[ME]"),
+        (Period("2012-01", freq="M"), "period[M]"),
         (Period("2012-02-01", freq="D"), "period[D]"),
         (
             Timestamp("2011-01-01", tz="US/Eastern"),