API: Disallow dtypes w/o frequency when casting
Previously deprecated for `Series` constructor and the `.astype` method. Now being enforced.

xref #15987.
Hello @gfyoung! Thanks for submitting the PR.

 - There are no PEP8 issues in the file [`pandas/core/dtypes/cast.py`](https://github.com/gfyoung/pandas/blob/4e266ab0043e39e145bcb80b01cfa0b36d5d7498/pandas/core/dtypes/cast.py) !

 - There are no PEP8 issues in the file [`pandas/tests/series/test_constructors.py`](https://github.com/gfyoung/pandas/blob/4e266ab0043e39e145bcb80b01cfa0b36d5d7498/pandas/tests/series/test_constructors.py) !

 - There are no PEP8 issues in the file [`pandas/tests/series/test_dtypes.py`](https://github.com/gfyoung/pandas/blob/4e266ab0043e39e145bcb80b01cfa0b36d5d7498/pandas/tests/series/test_dtypes.py) !

 # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/23392?src=pr&el=h1) Report
> Merging [#23392](https://codecov.io/gh/pandas-dev/pandas/pull/23392?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/62a15fa4071d0711b2b97771b981d527c1c4af10?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/23392/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/23392?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #23392      +/-   ##
==========================================
- Coverage   92.16%   92.16%   -0.01%     
==========================================
  Files         166      166              
  Lines       51224    51222       -2     
==========================================
- Hits        47212    47208       -4     
- Misses       4012     4014       +2
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.59% <100%> (-0.01%)` | :arrow_down: |
| #single | `42.22% <20%> (-0.01%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/23392?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/dtypes/cast.py](https://codecov.io/gh/pandas-dev/pandas/pull/23392/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZHR5cGVzL2Nhc3QucHk=) | `88.92% <100%> (-0.36%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/23392?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/23392?src=pr&el=footer). Last update [62a15fa...4c0d882](https://codecov.io/gh/pandas-dev/pandas/pull/23392?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 @jbrockmendel can you have a look see. I remember this now, so this lgtm. @jreback : Rebased, and all is green.  PTAL. thanks @gfyoung  Small comments aside, LGTM. @gfyoung if you can follow up with those comments in another PR