MAINT: Expand lint for *.py
Expand lint to include all Python files, excluding the directories `pandas/rpy` (deprecated and soon to be removed) and `pandas/src` (C code).

are flake8 options not strict enough? IOW why were these not caught before?
 ## [Current coverage](https://codecov.io/gh/pandas-dev/pandas/pull/14516?src=pr) is 85.26% (diff: 91.66%)

> Merging [#14516](https://codecov.io/gh/pandas-dev/pandas/pull/14516?src=pr) into [master](https://codecov.io/gh/pandas-dev/pandas/branch/master?src=pr) will not change coverage

``` diff
@@             master     #14516   diff @@
==========================================
  Files           140        140          
  Lines         50672      50672          
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
  Hits          43207      43207          
  Misses         7465       7465          
  Partials          0          0          
```

> Powered by [Codecov](https://codecov.io?src=pr). Last update [d7fb5bd...e7429aa](https://codecov.io/gh/pandas-dev/pandas/compare/d7fb5bd310edcab9875c1f9339b62e92baae8291...e7429aafce4d3136d71bdbbb60caf68792b0b520?src=pr)
 @jreback : TBH, I have no idea.  I didn't really change anything with the command flags.  But if it works, that's work counts I guess. :smile: 
 @jreback : Is this good to merge since Travis is passing?
 @jorisvandenbossche : Any reason why you moved it to 0.20.0?  This isn't an API change.
 It doesn't matter to merge now or not, that just indicates that it is not essential to backport
 Ah, okay.  Good to know!
 @gfyoung thanks!
