REF: Changed ExtensionDtype inheritance
`is_extension_array_dtype(dtype)` was incorrect for dtypes that haven't
implemented the new interface yet. This is because they indirectly subclassed
ExtensionDtype.

This PR changes the hierarchy so that PandasExtensionDtype doesn't subclass
ExtensionDtype. As we implement the interface, like Categorical, we'll add
ExtensionDtype as a base class.

Before:

```
DatetimeTZDtype <- PandasExtensionDtype <- ExtensionDtype (wrong)
CategoricalDtype <- PandasExtensionDtype <- ExtensionDtype (right)

After:

DatetimeTZDtype <- PandasExtensionDtype
                                        \
                                         - _DtypeOpsMixin
                                        /
                   ExtensionDtype ------

CategoricalDtype - PandasExtensionDtype -
                \                        \
                 \                        -_DtypeOpsMixin
                  \                      /
                   ExtensionDtype -------

```

Once all our extension dtypes have implemented the interface we can go back
to the simple, linear inheritance structure.
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/20363?src=pr&el=h1) Report
> Merging [#20363](https://codecov.io/gh/pandas-dev/pandas/pull/20363?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/87f565462326ec0a74d580bc8bafbd3273b21fd5?src=pr&el=desc) will **increase** coverage by `0.01%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/20363/graphs/tree.svg?width=650&height=150&src=pr&token=eZ4WkYLtcO)](https://codecov.io/gh/pandas-dev/pandas/pull/20363?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #20363      +/-   ##
==========================================
+ Coverage   91.78%   91.79%   +0.01%     
==========================================
  Files         152      152              
  Lines       49196    49186      -10     
==========================================
- Hits        45153    45152       -1     
+ Misses       4043     4034       -9
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.18% <100%> (+0.01%)` | :arrow_up: |
| #single | `41.83% <88.46%> (+0.01%)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/20363?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/dtypes/dtypes.py](https://codecov.io/gh/pandas-dev/pandas/pull/20363/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZHR5cGVzL2R0eXBlcy5weQ==) | `95.9% <100%> (ø)` | :arrow_up: |
| [pandas/core/dtypes/base.py](https://codecov.io/gh/pandas-dev/pandas/pull/20363/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZHR5cGVzL2Jhc2UucHk=) | `91.89% <100%> (+0.22%)` | :arrow_up: |
| [pandas/core/dtypes/common.py](https://codecov.io/gh/pandas-dev/pandas/pull/20363/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZHR5cGVzL2NvbW1vbi5weQ==) | `94.64% <100%> (ø)` | :arrow_up: |
| [pandas/core/dtypes/cast.py](https://codecov.io/gh/pandas-dev/pandas/pull/20363/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZHR5cGVzL2Nhc3QucHk=) | `87.68% <100%> (ø)` | :arrow_up: |
| [pandas/core/internals.py](https://codecov.io/gh/pandas-dev/pandas/pull/20363/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW50ZXJuYWxzLnB5) | `95.54% <100%> (ø)` | :arrow_up: |
| [pandas/core/indexes/datetimes.py](https://codecov.io/gh/pandas-dev/pandas/pull/20363/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9kYXRldGltZXMucHk=) | `95.64% <0%> (ø)` | :arrow_up: |
| [pandas/core/generic.py](https://codecov.io/gh/pandas-dev/pandas/pull/20363/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZ2VuZXJpYy5weQ==) | `95.85% <0%> (ø)` | :arrow_up: |
| [pandas/core/series.py](https://codecov.io/gh/pandas-dev/pandas/pull/20363/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvc2VyaWVzLnB5) | `93.84% <0%> (ø)` | :arrow_up: |
| [pandas/core/indexes/datetimelike.py](https://codecov.io/gh/pandas-dev/pandas/pull/20363/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9kYXRldGltZWxpa2UucHk=) | `96.72% <0%> (ø)` | :arrow_up: |
| [pandas/errors/\_\_init\_\_.py](https://codecov.io/gh/pandas-dev/pandas/pull/20363/diff?src=pr&el=tree#diff-cGFuZGFzL2Vycm9ycy9fX2luaXRfXy5weQ==) | `92.3% <0%> (ø)` | :arrow_up: |
| ... and [4 more](https://codecov.io/gh/pandas-dev/pandas/pull/20363/diff?src=pr&el=tree-more) | |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/20363?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/20363?src=pr&el=footer). Last update [87f5654...da2451e](https://codecov.io/gh/pandas-dev/pandas/pull/20363?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 Added tests for each extension dtype. Will have to move from `not` to `is` as we migrate them.