Remove deprecated warn kwarg for matplotlib use
The warn param has been deprecated in 3.2.1: https://matplotlib.org/3.2.1/api/prev_api_changes/api_changes_3.2.0.html#matplotlib-use

On the most recent matplotlib, this is failing for me with 

```
TypeError: use() got an unexpected keyword argument 'warn'
```

restarting failed Windows py37_np18 job

```
=========================== short test summary info ===========================
FAILED pandas/tests/io/parser/test_common.py::test_chunks_have_consistent_numerical_type[python]
= 1 failed, 72698 passed, 1743 skipped, 1053 xfailed, 58 warnings in 719.92s (0:11:59) =
``` Thanks! Thanks!