De-privatize tslibs functions
Related: an item on the tslibs TODO list, #21698.
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/21714?src=pr&el=h1) Report
> Merging [#21714](https://codecov.io/gh/pandas-dev/pandas/pull/21714?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/7cd2679982138e1e7f9f1a759827647e6e521d43?src=pr&el=desc) will **decrease** coverage by `<.01%`.
> The diff coverage is `69.23%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/21714/graphs/tree.svg?height=150&width=650&token=eZ4WkYLtcO&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/21714?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #21714      +/-   ##
==========================================
- Coverage    91.9%    91.9%   -0.01%     
==========================================
  Files         158      158              
  Lines       49690    49688       -2     
==========================================
- Hits        45670    45668       -2     
  Misses       4020     4020
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.28% <69.23%> (-0.01%)` | :arrow_down: |
| #single | `41.95% <0%> (-0.01%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/21714?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/indexes/accessors.py](https://codecov.io/gh/pandas-dev/pandas/pull/21714/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9hY2Nlc3NvcnMucHk=) | `90.09% <ø> (-0.1%)` | :arrow_down: |
| [pandas/core/indexes/datetimelike.py](https://codecov.io/gh/pandas-dev/pandas/pull/21714/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9kYXRldGltZWxpa2UucHk=) | `96.72% <100%> (ø)` | :arrow_up: |
| [pandas/core/indexes/period.py](https://codecov.io/gh/pandas-dev/pandas/pull/21714/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9wZXJpb2QucHk=) | `92.57% <62.5%> (-0.02%)` | :arrow_down: |
| [pandas/tseries/frequencies.py](https://codecov.io/gh/pandas-dev/pandas/pull/21714/diff?src=pr&el=tree#diff-cGFuZGFzL3RzZXJpZXMvZnJlcXVlbmNpZXMucHk=) | `95.9% <75%> (ø)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/21714?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/21714?src=pr&el=footer). Last update [7cd2679...fd393df](https://codecov.io/gh/pandas-dev/pandas/pull/21714?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 thanks @jbrockmendel 