MAINT: flake8 *.pyx files
closes #12995 

flake8-ed *.pyx files and fixed errors.

Removed the E226 check because that inhibits pointers (e.g. char*). In addition, the check is not even universally accepted in Python.

## [Current coverage](https://codecov.io/gh/pydata/pandas/pull/14147?src=pr) is 85.26% (diff: 100%)

> No coverage report found for **master** at 3110a72.
> 
> Powered by [Codecov](https://codecov.io?src=pr). Last update [3110a72...5677916](https://codecov.io/gh/pydata/pandas/compare/3110a72b5388f60de2cd287359e020f7c135b5e5...56779165b111e8de7720a2e159595f8743c61d58?src=pr)
 not sure what u mean E226 is not universally accepted

pls turn it back on
 ok, forget about E226, its not so easy to remove.

can you add E128 back to the `.pxi.in` as well.
 @jreback : I'm actually surprised that all of these `flake8` tests passed. :smile: In any case, do you think we can postpone the `pxi.in` changes until a separate PR?  This `pyx` change is massive enough as it is.
 no this needs fixing here as u edited the .pyx for algos and join which are auto generated and will revert when the pxi.in are changed 
 @jreback : Ah, fair enough.  Will fix then.
 @jreback : On second look, what do you mean that `algos.pyx` is auto-generated?  I believe you mean `algos*.pxi` are auto-generated.  Those are not the same files.
 right the .pxi are included at cythonization time
so if u only edited .pyx then ok
 @jreback : Nice!  You got Appveyor to work and now block this PR :smile: :angry: 

At least Appveyor is not failing because of my PR.  Need to fix that 3.5 failure with the locale.
 oh that's great!

and I stole from numpy a line in appveyor.yml
it cancels the previous if u push again :)
unlike Travis 
 thanks!
