pandas plotting - matplotlib 3.2.0 deprecation warning for rowNum/colNum attributes
With pandas 1.0.1 and matplotlib 3.2.0 in a conda Python 3.7.6 environment, I'm getting warnings about deprecation of rowNum and colNum attributes when using some pandas plotting routines.

Environment:
```
c> conda list "python|pandas|matplotlib|numpy|conda"
# packages in environment at C:\Users\me\Miniconda3\envs\forge:
#
# Name                    Version                   Build  Channel
conda                     4.8.2                    py37_0
conda-package-handling    1.6.0            py37h62dcd97_0
ipython                   7.13.0           py37h5ca1d4c_0    conda-forge
ipython_genutils          0.2.0                    py37_0
matplotlib                3.2.0                         1    conda-forge
matplotlib-base           3.2.0            py37h2981e6d_1    conda-forge
msys2-conda-epoch         20160418                      1
numpy                     1.18.1           py37h93ca92e_0
numpy-base                1.18.1           py37hc3f5095_1
numpydoc                  0.9.2                      py_0
pandas                    1.0.1            py37h47e9c7a_0
python                    3.7.6                h60c2a47_2
python-dateutil           2.8.1                      py_0
python-jsonrpc-server     0.3.4                      py_0
python-language-server    0.31.7                   py37_0
python_abi                3.7                     1_cp37m    conda-forge
```
Warnings:
```
C:\Users\me\Miniconda3\envs\forge\lib\site-packages\pandas\plotting\_matplotlib\tools.py:298: MatplotlibDeprecationWarning: 
The rowNum attribute was deprecated in Matplotlib 3.2 and will be removed two minor releases later. Use ax.get_subplotspec().rowspan.start instead.
  layout[ax.rowNum, ax.colNum] = ax.get_visible()
C:\Users\me\Miniconda3\envs\forge\lib\site-packages\pandas\plotting\_matplotlib\tools.py:298: MatplotlibDeprecationWarning: 
The colNum attribute was deprecated in Matplotlib 3.2 and will be removed two minor releases later. Use ax.get_subplotspec().colspan.start instead.
  layout[ax.rowNum, ax.colNum] = ax.get_visible()
```

The above warnings were generated when running this code:

```
iris.plot(subplots=True)
plt.suptitle('Iris data by index', x=0.5, y=0.925)

# Display the plot
plt.show()
```

and this code:

```
df = pd.DataFrame(np.random.randn(250, 5), columns=['A','B','C','D', 'class'])
df['class'] = pd.Series(np.random.randint(0,2, 250))
cmap_tab10 = plt.get_cmap("tab10")
pd.plotting.scatter_matrix(df.iloc[:,0:4], color=cmap_tab10(df['class']), alpha=0.2)
for ax in plt.gcf().axes:
    plt.sca(ax)
    plt.ylabel(ax.get_ylabel(), rotation=0, labelpad=8, ha="right")
    plt.grid(False)
plt.show()
```
In both of the above cases, the plots did draw as expected.  