MAINT: Ignore .pxi files
All `.pxi` files have the warning to not edit directly, so why not have Git ignore any changes made there?

In addition, this PR deletes all of the `.pxi` files from the codebase because they are automatically generated by the `.pxi.in` files

if you did this and don't check them in.

the a new clone & build will generate them, so that's ok.

when you change a ``.in`` they will get regenerated, so that seems ok.

when you switch branches what happens? (e.g. modify something in a .in in one branch), switch to another
make should just regen the files you need. does that work? @jreback : Yes, it does. ok so in this PR, remove all of the .pxi as well. ## [Current coverage](https://codecov.io/gh/pandas-dev/pandas/pull/14723?src=pr) is 85.21% (diff: 100%)
> Merging [#14723](https://codecov.io/gh/pandas-dev/pandas/pull/14723?src=pr) into [master](https://codecov.io/gh/pandas-dev/pandas/branch/master?src=pr) will increase coverage by **<.01%**

```diff
@@             master     #14723   diff @@
==========================================
  Files           143        143          
  Lines         50804      50804          
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
+ Hits          43292      43293     +1   
+ Misses         7512       7511     -1   
  Partials          0          0          
```

> Powered by [Codecov](https://codecov.io?src=pr). Last update [75b606a...fe0c410](https://codecov.io/gh/pandas-dev/pandas/compare/75b606abad51762b3faace1bbfa0a8d2241dc297...fe0c4101d3982152a09c30aabbef00c410fb605a?src=pr) Travis OSX keeps breaking because it cannot seem to hit the `github.com` endpoint when cloning the repository, which is unrelated to this PR.  Silencing testing for now until that resolves itself. cc @sinhrks @chris-b1  @jreback , @jorisvandenbossche : Travis is behaving again, so this should be ready to merge. can  you confirm that that travis caching still works?  I've added these files only for travis caching, so it's good to remove if caching can still work. @sinhrks , @jreback : I don't see any issues with caching on Travis, so I think this should be good to go. thanks! let's keep an eye on the caching though