BUG: MatplotlibDeprecationWarning:  The is_first_col function was deprecated in Matplotlib 3.4
```python
import pandas as pd
from matplotlib import pyplot as plt

series = [pd.Series([1,2,3,4]) for _ in range(4)]

nrow, ncol = 2, 2
fig, axes = plt.subplots(nrow, ncol)
axes = axes.reshape(-1)
for i, s in enumerate(series):
    s.plot(ax=axes[i])

plt.tight_layout()
```

#### Problem description

After executing the previous code the following message from Matplotlib is shown:

```my_personal_folder/lib/python3.8/site-packages/pandas/plotting/_matplotlib/tools.py:400: MatplotlibDeprecationWarning: 
The is_first_col function was deprecated in Matplotlib 3.4 and will be removed two minor releases later. Use ax.get_subplotspec().is_first_col() instead.
  if ax.is_first_col():
```
