CLN: non-nano follow-ups (pandas-dev#47682)

Author: jbrockmendel

pandas/_libs/tslib.pyx

@@ -152,7 +152,7 @@ def format_array_from_datetime(
         # a format based on precision
         basic_format = format is None
         if basic_format:
-            reso_obj = get_resolution(values, reso=reso)
+            reso_obj = get_resolution(values, tz=tz, reso=reso)
             show_ns = reso_obj == Resolution.RESO_NS
             show_us = reso_obj == Resolution.RESO_US
             show_ms = reso_obj == Resolution.RESO_MS

pandas/_libs/tslibs/conversion.pyx

@@ -144,9 +144,13 @@ cpdef inline (int64_t, int) precision_from_unit(str unit):
         NPY_DATETIMEUNIT reso = abbrev_to_npy_unit(unit)
 
     if reso == NPY_DATETIMEUNIT.NPY_FR_Y:
+        # each 400 years we have 97 leap years, for an average of 97/400=.2425
+        #  extra days each year. We get 31556952 by writing
+        #  3600*24*365.2425=31556952
         m = 1_000_000_000 * 31556952
         p = 9
     elif reso == NPY_DATETIMEUNIT.NPY_FR_M:
+        # 2629746 comes from dividing the "Y" case by 12.
         m = 1_000_000_000 * 2629746
         p = 9
     elif reso == NPY_DATETIMEUNIT.NPY_FR_W:

pandas/_libs/tslibs/vectorized.pyi

@@ -42,5 +42,5 @@ def ints_to_pydatetime(
 def tz_convert_from_utc(
     stamps: npt.NDArray[np.int64],
     tz: tzinfo | None,
-    reso: int = ...,
+    reso: int = ...,  # NPY_DATETIMEUNIT
 ) -> npt.NDArray[np.int64]: ...

pandas/core/indexes/base.py

@@ -6897,6 +6897,7 @@ def insert(self, loc: int, item) -> Index:
 
         # Use self._constructor instead of Index to retain NumericIndex GH#43921
         # TODO(2.0) can use Index instead of self._constructor
+        #  Check if doing so fixes GH#47071
         return self._constructor._with_infer(new_values, name=self.name)
 
     def drop(