Fixed PeriodIndex._shallow_copy for i8

[TomAugspurger]

Closes #24391

cc @jschendel

[jreback]

lgtm.

[codecov]

Codecov Report

Merging #24604 into master will decrease coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master   #24604      +/-   ##
==========================================
- Coverage   43.05%   43.05%   -0.01%     
==========================================
  Files         166      166              
  Lines       52471    52482      +11     
==========================================
+ Hits        22593    22595       +2     
- Misses      29878    29887       +9

Flag	Coverage Δ
#single	43.05% <100%> (-0.01%)	⬇️

Impacted Files	Coverage Δ
pandas/core/indexes/period.py	36.77% <100%> (+0.02%)	⬆️
pandas/core/internals/blocks.py	51.93% <0%> (-1%)	⬇️
pandas/util/testing.py	50.73% <0%> (-0.93%)	⬇️
pandas/core/dtypes/dtypes.py	73.57% <0%> (-0.5%)	⬇️
pandas/core/indexes/datetimelike.py	57.55% <0%> (-0.22%)	⬇️
pandas/core/indexes/category.py	50.8% <0%> (-0.03%)	⬇️
pandas/core/strings.py	33% <0%> (ø)	⬆️
pandas/core/internals/__init__.py	100% <0%> (ø)	⬆️
pandas/core/arrays/timedeltas.py	41.53% <0%> (ø)	⬆️
pandas/core/arrays/datetimes.py	65.78% <0%> (ø)	⬆️
... and 7 more

---

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[codecov]

Codecov Report

Merging #24604 into master will decrease coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master   #24604      +/-   ##
==========================================
- Coverage   92.37%   92.37%   -0.01%     
==========================================
  Files         166      166              
  Lines       52471    52470       -1     
==========================================
- Hits        48471    48469       -2     
- Misses       4000     4001       +1

Flag	Coverage Δ
#multiple	90.8% <100%> (-0.01%)	⬇️
#single	43.06% <25%> (ø)	⬆️

Impacted Files	Coverage Δ
pandas/core/resample.py	97.23% <100%> (ø)	⬆️
pandas/core/indexes/period.py	92.06% <100%> (-0.06%)	⬇️
pandas/util/testing.py	88% <0%> (-0.1%)	⬇️

---

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[TomAugspurger]

Had to update a few places using this.

One in resample. We were using this for PI/TDI/DTI to change the freq. It only worked since the object was empty.

The others were tests using the _shallow_copy API. Those have been updated to use asfreq for PI.

[TomAugspurger]

All green now.

[jreback]

thanks @TomAugspurger

[jschendel]

thanks @TomAugspurger