PERF: benchmarks are hitting .ix & other warnings
https://dev.azure.com/pandas-dev/pandas/_build/results?buildId=13969 (off of a PR)

shows quite a number of warnings of the form; we should either suppress or remove the benchmark.

 ```
24.97%] ··· io.msgpack.MSGPack.time_read_msgpack                      25.2±0ms
[ 24.97%] ···· /home/vsts/work/1/s/asv_bench/benchmarks/io/msgpack.py:18: FutureWarning: to_msgpack is deprecated and will be removed in a future version.
               It is recommended to use pyarrow for on-the-wire transmission of pandas objects.
                 self.df.to_msgpack(self.fname)
               /home/vsts/miniconda3/envs/pandas-dev/lib/python3.7/timeit.py:6: FutureWarning: The read_msgpack is deprecated and will be removed in a future version.
               It is recommended to use pyarrow for on-the-wire transmission of pandas objects.
                 times.  See also Tim Peters' introduction to the Algorithms chapter in
```

```
  6.83%] ···· For parameters: 0
               /home/vsts/work/1/s/asv_bench/benchmarks/frame_methods.py:329: FutureWarning: 
               .ix is deprecated. Please use
               .loc for label based indexing or
               .iloc for positional indexing
               
               See the documentation here:
               http://pandas.pydata.org/pandas-docs/stable/user_guide/indexing.html#ix-indexer-is-deprecated
                 self.df.ix[50:1000, 20:50] = np.nan
               /home/vsts/work/1/s/asv_bench/benchmarks/frame_methods.py:330: FutureWarning: 
               .ix is deprecated. Please use
               .loc for label based indexing or
               .iloc for positional indexing
               
               See the documentation here:
               http://pandas.pydata.org/pandas-docs/stable/user_guide/indexing.html#ix-indexer-is-deprecated
                 self.df.ix[2000:3000] = np.nan
               /home/vsts/work/1/s/asv_bench/benchmarks/frame_methods.py:331: FutureWarning: 
               .ix is deprecated. Please use
               .loc for label based indexing or
               .iloc for positional indexing
               
               See the documentation here:
               http://pandas.pydata.org/pandas-docs/stable/user_guide/indexing.html#ix-indexer-is-deprecated
                 self.df.ix[:, 60:70] = np.nan
               
               For parameters: 1
               /home/vsts/work/1/s/asv_bench/benchmarks/frame_methods.py:329: FutureWarning: 
               .ix is deprecated. Please use
               .loc for label based indexing or
               .iloc for positional indexing
               
               See the documentation here:
               http://pandas.pydata.org/pandas-docs/stable/user_guide/indexing.html#ix-indexer-is-deprecated
                 self.df.ix[50:1000, 20:50] = np.nan
               /home/vsts/work/1/s/asv_bench/benchmarks/frame_methods.py:330: FutureWarning: 
               .ix is deprecated. Please use
               .loc for label based indexing or
               .iloc for positional indexing
               
               See the documentation here:
               http://pandas.pydata.org/pandas-docs/stable/user_guide/indexing.html#ix-indexer-is-deprecated
                 self.df.ix[2000:3000] = np.nan
               /home/vsts/work/1/s/asv_bench/benchmarks/frame_methods.py:331: FutureWarning: 
               .ix is deprecated. Please use
               .loc for label based indexing or
               .iloc for positional indexing
               
               See the documentation here:
               http://pandas.pydata.org/pandas-docs/stable/user_guide/indexing.html#ix-indexer-is-deprecated
                 self.df.ix[:, 60:70] = np.nan

[  6.89%] ··· frame_methods.Count.time_count_level_multi                      ok
[  6.89%] ··· ====== ==========
               axis            
              ------ ----------
                0     78.6±0ms 
                1     114±0ms  
              ====== ==========

[  6.89%] ···· For parameters: 0
               /home/vsts/work/1/s/asv_bench/benchmarks/frame_methods.py:329: FutureWarning: 
               .ix is deprecated. Please use
               .loc for label based indexing or
               .iloc for positional indexing
               
               See the documentation here:
               http://pandas.pydata.org/pandas-docs/stable/user_guide/indexing.html#ix-indexer-is-deprecated
                 self.df.ix[50:1000, 20:50] = np.nan
               /home/vsts/work/1/s/asv_bench/benchmarks/frame_methods.py:330: FutureWarning: 
               .ix is deprecated. Please use
               .loc for label based indexing or
               .iloc for positional indexing
               
               See the documentation here:
               http://pandas.pydata.org/pandas-docs/stable/user_guide/indexing.html#ix-indexer-is-deprecated
                 self.df.ix[2000:3000] = np.nan
               /home/vsts/work/1/s/asv_bench/benchmarks/frame_methods.py:331: FutureWarning: 
               .ix is deprecated. Please use
               .loc for label based indexing or
               .iloc for positional indexing
               
               See the documentation here:
               http://pandas.pydata.org/pandas-docs/stable/user_guide/indexing.html#ix-indexer-is-deprecated
                 self.df.ix[:, 60:70] = np.nan
               
               For parameters: 1
               /home/vsts/work/1/s/asv_bench/benchmarks/frame_methods.py:329: FutureWarning: 
               .ix is deprecated. Please use
               .loc for label based indexing or
               .iloc for positional indexing
               
               See the documentation here:
               http://pandas.pydata.org/pandas-docs/stable/user_guide/indexing.html#ix-indexer-is-deprecated
                 self.df.ix[50:1000, 20:50] = np.nan
               /home/vsts/work/1/s/asv_bench/benchmarks/frame_methods.py:330: FutureWarning: 
               .ix is deprecated. Please use
               .loc for label based indexing or
               .iloc for positional indexing
               
               See the documentation here:
               http://pandas.pydata.org/pandas-docs/stable/user_guide/indexing.html#ix-indexer-is-deprecated
                 self.df.ix[2000:3000] = np.nan
               /home/vsts/work/1/s/asv_bench/benchmarks/frame_methods.py:331: FutureWarning: 
               .ix is deprecated. Please use
               .loc for label based indexing or
               .iloc for positional indexing
               
               See the documentation here:
               http://pandas.pydata.org/pandas-docs/stable/user_guide/indexing.html#ix-indexer-is-deprecated
                 self.df.ix[:, 60:70] = np.nan

[  6.96%] ··· frame_methods.Describe.time_dataframe_describe             133±0ms
[  7.02%] ··· frame_methods.Describe.time_series_describe               44.2±0ms
```