BUG: MatplotlibDeprecationWarning is_last_row function was deprecated in Matplotlib 3.4 
Seeing this warning while using

```
pyfolio.create_returns_tear_sheet(stock_rets)
```

```
.../lib/python3.8/site-packages/pandas/plotting/_matplotlib/tools.py:418: MatplotlibDeprecationWarning: 
The is_last_row function was deprecated in Matplotlib 3.4 and will be removed two minor releases later. Use ax.get_subplotspec().is_last_row() instead.
  if ax.is_last_row():
.../lib/python3.8/site-packages/pandas/plotting/_matplotlib/tools.py:418: MatplotlibDeprecationWarning: 
The is_last_row function was deprecated in Matplotlib 3.4 and will be removed two minor releases later. Use ax.get_subplotspec().is_last_row() instead.
  if ax.is_last_row():
```

change tools.py:418 as suggested did fix the warning.

```
            except IndexError:
                # if gridspec is used, ax.rowNum and ax.colNum may different
                # from layout shape. in this case, use last_row logic
                for ax in axarr:
                    if ax.get_subplotspecis_last_row():
                        continue
                    if sharex or _has_externally_shared_axis(ax, "x"):
                        _remove_labels_from_axis(ax.xaxis)
````
