DOC: Fix flake8 issue in whatsnew files
In some of the whatsnew files we are using implicit imports. For example in "whatsnew/v0.11.0.rst"
we import pandas `from pandas import *` implicitly.  We instantiate using the implicitly imported class `DataFrame` later in the code. 

`df1 = DataFrame(randn(8, 1), columns = ['A'], dtype = 'float32')`
`df1`

The preferred approach is to use explicit import as `import pandas as pd` and replace the call to `DataFrame` with `pd.DateFrame`.

**Note:** 

1. `import pandas as pd` is part of the header, so it is imported by default. In that case just replacing `DataFrame` with `pd.DateFrame` and removing `from pandas import *` should be enough. 
2. Many whatnew files are using the preferred approach of explicit calling `pd.*`. However, the import statement of `from pandas import *` is included by default. In such case, just remove the import statement. 

cc: @datapythonista 

### Status of whatsnew files 
| File Name      | Implicit Imports | Flake8-rst |
| ------------- | ------------- | ------------|
| v0.24.0.rst  |  Pass | Pass: #24217 |
| v0.23.4.rst  |  Pass | Pass |
| v0.23.3.rst |   Pass| Pass #24273 |
| v0.23.2.rst |   Pass | Pass |
| v0.23.1.rst  |  Pass: #24199 | Pass #24273 |
| v0.23.0.rst | Pass | Pass: #24217 |
| v0.22.0.rst | Pass | Pass: #24217 |
| v0.21.1.rst | Pass | Pass |
| v0.21.0.rst | Pass | Pass: #24217 |
| v0.20.3.rst| Pass | Pass|
| v0.20.2.rst| Pass | Pass|
|v0.20.0.rst | Pass | Pass: #24236|
|v0.19.2.rst | Pass | Pass |
| v0.19.1.rst| Pass | Pass |
| v0.19.0.rst| Pass #24236 | Pass #24236 |
|v0.18.1.rst| Pass | Pass|
|v0.18.0.rst| Pass #24233| Pass #24233|
|v0.17.1.rst|Pass #24235|Pass #24235 |
|v0.17.0.rst|Pass #24235|Pass #24235|
|v0.16.2.rst|Pass  #24338|Pass #24338|
|v0.16.1.rst|Pass  #24338|Pass  #24338|
|v0.16.0.rst|Pass  #24338|Pass  #24338|
|v0.15.2.rst|Pass #24635 |Pass #24635 |
|v0.15.1.rst|Pass #24635|Pass #24635|
|v0.15.0.rst|Pass #24635|Pass #24635|
|v0.14.1.rst|Pass #24253| Pass #24253|
|v0.14.0.rst| Pass #24253| Pass #24253|
|v0.13.1.rst|Pass #24322| Pass #24322|
|v0.13.0.rst| Pass #24322| Pass #24322|
|v0.12.0.rst| Pass #24309| Pass #24309 |
|v0.11.0.rst| Pass #24277 | Pass #24277|
|v0.10.1.rst|Pass #24277| Pass #24277|
|v0.10.0.rst| Pass #24277| Pass #24277|
|v0.7*-v0.9*|Pass #24273 |Pass #24273 |
|v0.4*-v0.6*| Pass | Pass|