CLN: ASV replace
- Flake8'd and removed star imports

For the `ReplaceDict` benchmark I reduced the number of samples from 10^6 to 10^5 because it was taking a significant amount of time, but 10^5 samples should still be representative of a large sample performance.

```
asv dev -b ^replace
· Discovering benchmarks
· Running 4 total benchmarks (1 commits * 1 environments * 4 benchmarks)
[  0.00%] ·· Building for existing-py_home_matt_anaconda_envs_pandas_dev_bin_python
[  0.00%] ·· Benchmarking existing-py_home_matt_anaconda_envs_pandas_dev_bin_python
[ 25.00%] ··· Running replace.Convert.time_replace                                                               ok
[ 25.00%] ···· 
               ============ =========== ===========
               --                 replace_data     
               ------------ -----------------------
                contructor   Timestamp   Timedelta 
               ============ =========== ===========
                DataFrame      411ms       409ms   
                  Series       349ms       349ms   
               ============ =========== ===========

[ 50.00%] ··· Running replace.NaN.time_fillna                                                                    ok
[ 50.00%] ···· 
               ========= ========
                inplace          
               --------- --------
                  True    6.42ms 
                 False    15.0ms 
               ========= ========

[ 75.00%] ··· Running replace.NaN.time_replace                                                                   ok
[ 75.00%] ···· 
               ========= ========
                inplace          
               --------- --------
                  True    7.09ms 
                 False    23.6ms 
               ========= ========

[100.00%] ··· Running replace.ReplaceDict.time_replace_series                                                    ok
[100.00%] ···· 
               ========= =======
                inplace         
               --------- -------
                  True    7.43s 
                 False    7.48s 
               ========= =======
```
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18833?src=pr&el=h1) Report
> Merging [#18833](https://codecov.io/gh/pandas-dev/pandas/pull/18833?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/b6a7cc9d4686f62596de031c9cc4a1d62d12c52e?src=pr&el=desc) will **decrease** coverage by `0.01%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/18833/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/18833?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #18833      +/-   ##
==========================================
- Coverage   91.64%   91.62%   -0.02%     
==========================================
  Files         154      154              
  Lines       51408    51408              
==========================================
- Hits        47113    47104       -9     
- Misses       4295     4304       +9
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.49% <ø> (ø)` | :arrow_up: |
| #single | `40.83% <ø> (-0.12%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/18833?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/18833/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/18833/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.68% <0%> (-0.11%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18833?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/18833?src=pr&el=footer). Last update [b6a7cc9...5b97df9](https://codecov.io/gh/pandas-dev/pandas/pull/18833?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 Changed the class name from `NaN` to `Fillna` thanks @mroeschke 