 BUG: treat nan-objects the same way float64-nans are treated - all na…  
…ns are from the same equivalence class (GH22119)

- [x] closes #22119
- [x] closes #22148
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

##### asv_benchmark:

```
asv continuous -f 1.01 upstream/master HEAD -b ^series_methods -b ^algorithms
....                                                           
       before           after         ratio
     [d30c4a06]       [e583a0c9]
-      2.14±0.04s         672±10μs     0.00  series_methods.IsInForObjects.time_isin_nans

SOME BENCHMARKS HAVE CHANGED SIGNIFICANTLY.
```

`series_methods.IsInForObjects.time_isin_nans` is a new test showing a test case which leads to `O(n^2)` for hashmap generation with the current implementation. # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/22207?src=pr&el=h1) Report
> Merging [#22207](https://codecov.io/gh/pandas-dev/pandas/pull/22207?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/776fed3ab63d74ddef6e5af1a702b10c2a30bbb6?src=pr&el=desc) will **increase** coverage by `<.01%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/22207/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/22207?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #22207      +/-   ##
==========================================
+ Coverage   92.06%   92.06%   +<.01%     
==========================================
  Files         169      169              
  Lines       50694    50698       +4     
==========================================
+ Hits        46671    46676       +5     
+ Misses       4023     4022       -1
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.47% <ø> (ø)` | :arrow_up: |
| #single | `42.32% <ø> (-0.01%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/22207?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/tseries/offsets.py](https://codecov.io/gh/pandas-dev/pandas/pull/22207/diff?src=pr&el=tree#diff-cGFuZGFzL3RzZXJpZXMvb2Zmc2V0cy5weQ==) | `97.15% <0%> (ø)` | :arrow_up: |
| [pandas/io/common.py](https://codecov.io/gh/pandas-dev/pandas/pull/22207/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2NvbW1vbi5weQ==) | `70.65% <0%> (ø)` | :arrow_up: |
| [pandas/core/internals/blocks.py](https://codecov.io/gh/pandas-dev/pandas/pull/22207/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW50ZXJuYWxzL2Jsb2Nrcy5weQ==) | `94.45% <0%> (ø)` | :arrow_up: |
| [pandas/core/missing.py](https://codecov.io/gh/pandas-dev/pandas/pull/22207/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvbWlzc2luZy5weQ==) | `91.7% <0%> (+0.04%)` | :arrow_up: |
| [pandas/core/indexes/base.py](https://codecov.io/gh/pandas-dev/pandas/pull/22207/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9iYXNlLnB5) | `96.43% <0%> (+0.05%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/22207?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/22207?src=pr&el=footer). Last update [776fed3...5cc5e0f](https://codecov.io/gh/pandas-dev/pandas/pull/22207?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 thanks @realead nice patch
and thanks @mattip for testing