remove uses of (ts)?lib.(NaT|iNaT|Timestamp)
Since they are no longer defined there; instead import then from more appropriate places.
Hello @jbrockmendel! Thanks for submitting the PR.

 - There are no PEP8 issues in the file [`pandas/core/generic.py`](https://github.com/jbrockmendel/pandas/blob/5171cbb87430e98b87f9d6c7d2ac354584eeefdc/pandas/core/generic.py) !

 - There are no PEP8 issues in the file [`pandas/core/indexes/period.py`](https://github.com/jbrockmendel/pandas/blob/5171cbb87430e98b87f9d6c7d2ac354584eeefdc/pandas/core/indexes/period.py) !

 - There are no PEP8 issues in the file [`pandas/tests/dtypes/test_inference.py`](https://github.com/jbrockmendel/pandas/blob/5171cbb87430e98b87f9d6c7d2ac354584eeefdc/pandas/tests/dtypes/test_inference.py) !

 - There are no PEP8 issues in the file [`pandas/tests/frame/test_constructors.py`](https://github.com/jbrockmendel/pandas/blob/5171cbb87430e98b87f9d6c7d2ac354584eeefdc/pandas/tests/frame/test_constructors.py) !

 - There are no PEP8 issues in the file [`pandas/tests/indexes/datetimes/test_tools.py`](https://github.com/jbrockmendel/pandas/blob/5171cbb87430e98b87f9d6c7d2ac354584eeefdc/pandas/tests/indexes/datetimes/test_tools.py) !

 - There are no PEP8 issues in the file [`pandas/tests/indexes/period/test_ops.py`](https://github.com/jbrockmendel/pandas/blob/5171cbb87430e98b87f9d6c7d2ac354584eeefdc/pandas/tests/indexes/period/test_ops.py) !

 - There are no PEP8 issues in the file [`pandas/tests/scalar/period/test_period.py`](https://github.com/jbrockmendel/pandas/blob/5171cbb87430e98b87f9d6c7d2ac354584eeefdc/pandas/tests/scalar/period/test_period.py) !

 - There are no PEP8 issues in the file [`pandas/tests/series/indexing/test_datetime.py`](https://github.com/jbrockmendel/pandas/blob/5171cbb87430e98b87f9d6c7d2ac354584eeefdc/pandas/tests/series/indexing/test_datetime.py) !

 - There are no PEP8 issues in the file [`pandas/tests/series/test_dtypes.py`](https://github.com/jbrockmendel/pandas/blob/5171cbb87430e98b87f9d6c7d2ac354584eeefdc/pandas/tests/series/test_dtypes.py) !

 - There are no PEP8 issues in the file [`pandas/tests/series/test_internals.py`](https://github.com/jbrockmendel/pandas/blob/5171cbb87430e98b87f9d6c7d2ac354584eeefdc/pandas/tests/series/test_internals.py) !

 - There are no PEP8 issues in the file [`pandas/tests/series/test_replace.py`](https://github.com/jbrockmendel/pandas/blob/5171cbb87430e98b87f9d6c7d2ac354584eeefdc/pandas/tests/series/test_replace.py) !

 - There are no PEP8 issues in the file [`pandas/tests/tslibs/test_array_to_datetime.py`](https://github.com/jbrockmendel/pandas/blob/5171cbb87430e98b87f9d6c7d2ac354584eeefdc/pandas/tests/tslibs/test_array_to_datetime.py) !

 # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/23562?src=pr&el=h1) Report
> Merging [#23562](https://codecov.io/gh/pandas-dev/pandas/pull/23562?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/82120016e6af85afb052d95710e498a6dc58c1ce?src=pr&el=desc) will **not change** coverage.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/23562/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/23562?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #23562   +/-   ##
=======================================
  Coverage   92.24%   92.24%           
=======================================
  Files         161      161           
  Lines       51224    51224           
=======================================
  Hits        47254    47254           
  Misses       3970     3970
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.63% <100%> (ø)` | :arrow_up: |
| #single | `42.28% <57.14%> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/23562?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/generic.py](https://codecov.io/gh/pandas-dev/pandas/pull/23562/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZ2VuZXJpYy5weQ==) | `96.81% <100%> (ø)` | :arrow_up: |
| [pandas/core/indexes/period.py](https://codecov.io/gh/pandas-dev/pandas/pull/23562/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9wZXJpb2QucHk=) | `92.25% <100%> (ø)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/23562?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/23562?src=pr&el=footer). Last update [8212001...bf7249d](https://codecov.io/gh/pandas-dev/pandas/pull/23562?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 thanks!