PKG: Exclude data test files.
```
$ ls -lh dist
total 86552
-rw-r--r--  1 taugspurger  staff    10M Feb  4 14:12 pandas-0.23.0.dev0+218.g3f3b4e0bc-cp36-cp36m-macosx_10_12_x86_64.whl
-rw-r--r--  1 taugspurger  staff    12M Feb  4 14:12 pandas-0.23.0.dev0+218.g3f3b4e0bc.tar.gz
-rw-r--r--  1 taugspurger  staff   7.8M Feb  4 12:18 pandas-0.23.0.dev0+219.g4d77cd8e6-cp36-cp36m-macosx_10_12_x86_64.whl
-rw-r--r--  1 taugspurger  staff   9.5M Feb  4 14:11 pandas-0.23.0.dev0+219.g4d77cd8e6.tar.gz
```

Source: 12M -> 9.5M
Binary: 10M -> 7.8M

Need to do a bit more testing to make sure I didn't break anything.

And need to think about how to test this going forward to ensure we don't write tests that aren't skipped if a data file isn't present.

Closes https://github.com/pandas-dev/pandas/issues/19320
Closes https://github.com/pandas-dev/pandas/issues/21436