BUG: use more generic type inference for fast plotting
xref #15073

https://travis-ci.org/pandas-dev/pandas/jobs/190356982

was failing on tz-aware dtypes. these routines are the same in that they require datetime64ns (but any *also* accepts tz-aware).
@jorisvandenbossche  ## [Current coverage](https://codecov.io/gh/pandas-dev/pandas/pull/15094?src=pr) is 84.75% (diff: 90.90%)
> Merging [#15094](https://codecov.io/gh/pandas-dev/pandas/pull/15094?src=pr) into [master](https://codecov.io/gh/pandas-dev/pandas/branch/master?src=pr) will increase coverage by **<.01%**

```diff
@@             master     #15094   diff @@
==========================================
  Files           145        145          
  Lines         51232      51234     +2   
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
+ Hits          43420      43423     +3   
+ Misses         7812       7811     -1   
  Partials          0          0          
```

> Powered by [Codecov](https://codecov.io?src=pr). Last update [0fe491d...c64b56b](https://codecov.io/gh/pandas-dev/pandas/compare/0fe491db358204544814aa2371fed4ea947532ab...c64b56bc1357d7036b689c5787311d0386d0a53b?src=pr) ok, I fixed ``is_datetime64_ns_dtype`` to work with tz-aware (as _any_ does as well, this makes sense). let's see if anything else breaks. added a test as well. ok, this is actually a tiny public API change, but it was inconsistent / wrong before. @jorisvandenbossche  How safe is it to change this? Couldn't there be places in our code where `is_datetime64_ns_dtype` is that assumes it has no timezone?

Just as overview, is this correct?
- `is_datetime64_dtype` -> any resolution, no tz (pure numpy)
- `is_datetime64_any_dtype` -> any resolution, with or without tz
- `is_datetime64tz_dtype` -> ns? resolution, with tz (as other resolution with tz is not possible I think?)
- `is_datetime64_ns_dtype` -> ns resolution, with or without tz

The combination missing is "ns resolution, no tz (pure numpy)", which is actually what `is_datetime64_ns_dtype` was before. But maybe we don't need this? (as you can use the tz one to check it has *no* timezone) these functions are a bit redundant; the ``_ns_`` and ``_any_`` are barely used at all. so I don't think this is a big deal to change. Indeed, my search says that `_any_` is exactly used in one spot, and `_ns_` in three :-). So maybe rather something to clean-up? (but not for this PR) @jorisvandenbossche yeah I'd leave them for now, they *are* useful, even if not used :>

pushing in a sec @jorisvandenbossche  yes!