API: add dtype= option to python parser
- [x] part of #12686
- [x] tests added / passed
- [x] passes `git diff upstream/master | flake8 --diff`
- [x] whatsnew entry

Ultimately I'm working towards #8212 (types in excel parser), which should be pretty straightforward after this.

Right now the tests are moved from `c_parser_only.py`, may need to add some more
cc @gfyoung 

## [Current coverage](https://codecov.io/gh/pandas-dev/pandas/pull/14295?src=pr) is 85.22% (diff: 100%)
> Merging [#14295](https://codecov.io/gh/pandas-dev/pandas/pull/14295?src=pr) into [master](https://codecov.io/gh/pandas-dev/pandas/branch/master?src=pr) will increase coverage by **0.01%**

```diff
@@             master     #14295   diff @@
==========================================
  Files           143        143          
  Lines         50804      50833    +29   
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
+ Hits          43292      43324    +32   
+ Misses         7512       7509     -3   
  Partials          0          0          
```

> Powered by [Codecov](https://codecov.io?src=pr). Last update [75b606a...3abb0bd](https://codecov.io/gh/pandas-dev/pandas/compare/75b606abad51762b3faace1bbfa0a8d2241dc297...3abb0bd6e46e78557c1fd480ac173881dc5d530b?src=pr) @chris-b1 xref #14558. You say that `dtype` is ignored when `converters` is specified. In that case it would be nice to raise a warning about this if both are specified I think.
 I think I've got all the feedback so far worked in - any more comments?  @jorisvandenbossche, @gfyoung, @jreback 
 @chris-b1 : I'm putting together a PR to patch #14712.  As this directly relates to your PR, I would suggest waiting for mine to make sure the bug isn't an issue for you. @chris-b1 can you rebase this? Then this can be merged I think (depending on the status of @gfyoung new PR) Finally got back to this, rebased and updated docs - assuming tests still pass this should be good.  Will have to check, but I don't think this will conflict with #14717. @chris-b1 I merged #14717. This added a dtype test to `c_parser_only`, so added a commit to move this to the general dtype tests as well. Feel free to merge if you're ok with this and the tests are green. @chris-b1 Thanks! 
