Fixed PeriodIndex._shallow_copy for i8
Closes https://github.com/pandas-dev/pandas/issues/24391

cc @jschendel
lgtm. # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24604?src=pr&el=h1) Report
> Merging [#24604](https://codecov.io/gh/pandas-dev/pandas/pull/24604?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/25c1c38e83439c30b31c16d9032b19fcbe907e25?src=pr&el=desc) will **decrease** coverage by `<.01%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/24604/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/24604?src=pr&el=tree)

```diff
@@            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%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/24604?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/indexes/period.py](https://codecov.io/gh/pandas-dev/pandas/pull/24604/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9wZXJpb2QucHk=) | `36.77% <100%> (+0.02%)` | :arrow_up: |
| [pandas/core/internals/blocks.py](https://codecov.io/gh/pandas-dev/pandas/pull/24604/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW50ZXJuYWxzL2Jsb2Nrcy5weQ==) | `51.93% <0%> (-1%)` | :arrow_down: |
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/24604/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `50.73% <0%> (-0.93%)` | :arrow_down: |
| [pandas/core/dtypes/dtypes.py](https://codecov.io/gh/pandas-dev/pandas/pull/24604/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZHR5cGVzL2R0eXBlcy5weQ==) | `73.57% <0%> (-0.5%)` | :arrow_down: |
| [pandas/core/indexes/datetimelike.py](https://codecov.io/gh/pandas-dev/pandas/pull/24604/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9kYXRldGltZWxpa2UucHk=) | `57.55% <0%> (-0.22%)` | :arrow_down: |
| [pandas/core/indexes/category.py](https://codecov.io/gh/pandas-dev/pandas/pull/24604/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9jYXRlZ29yeS5weQ==) | `50.8% <0%> (-0.03%)` | :arrow_down: |
| [pandas/core/strings.py](https://codecov.io/gh/pandas-dev/pandas/pull/24604/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvc3RyaW5ncy5weQ==) | `33% <0%> (ø)` | :arrow_up: |
| [pandas/core/internals/\_\_init\_\_.py](https://codecov.io/gh/pandas-dev/pandas/pull/24604/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW50ZXJuYWxzL19faW5pdF9fLnB5) | `100% <0%> (ø)` | :arrow_up: |
| [pandas/core/arrays/timedeltas.py](https://codecov.io/gh/pandas-dev/pandas/pull/24604/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvYXJyYXlzL3RpbWVkZWx0YXMucHk=) | `41.53% <0%> (ø)` | :arrow_up: |
| [pandas/core/arrays/datetimes.py](https://codecov.io/gh/pandas-dev/pandas/pull/24604/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvYXJyYXlzL2RhdGV0aW1lcy5weQ==) | `65.78% <0%> (ø)` | :arrow_up: |
| ... and [7 more](https://codecov.io/gh/pandas-dev/pandas/pull/24604/diff?src=pr&el=tree-more) | |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24604?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24604?src=pr&el=footer). Last update [25c1c38...776aa90](https://codecov.io/gh/pandas-dev/pandas/pull/24604?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24604?src=pr&el=h1) Report
> Merging [#24604](https://codecov.io/gh/pandas-dev/pandas/pull/24604?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/25c1c38e83439c30b31c16d9032b19fcbe907e25?src=pr&el=desc) will **decrease** coverage by `<.01%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/24604/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/24604?src=pr&el=tree)

```diff
@@            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%)` | :arrow_down: |
| #single | `43.06% <25%> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/24604?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/resample.py](https://codecov.io/gh/pandas-dev/pandas/pull/24604/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvcmVzYW1wbGUucHk=) | `97.23% <100%> (ø)` | :arrow_up: |
| [pandas/core/indexes/period.py](https://codecov.io/gh/pandas-dev/pandas/pull/24604/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9wZXJpb2QucHk=) | `92.06% <100%> (-0.06%)` | :arrow_down: |
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/24604/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `88% <0%> (-0.1%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24604?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24604?src=pr&el=footer). Last update [25c1c38...776aa90](https://codecov.io/gh/pandas-dev/pandas/pull/24604?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 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. All green now. thanks @TomAugspurger  thanks @TomAugspurger