BUG: Fixes #36918 boxplots with matplotlib
Fixes #36918

I believe that the @TomAugspurger in #35393 found out that, when `sharex` or `sharey` are enabled, matplotlib counts the number of ticks of an axis in a subplot as the total number of ticks of the shared axis in all subplots. Since matplotlib became stricter with `set_xticks`, the proposed workaround was replicating the labels n times where n is "total number of ticks"/"number of ticks of this subplot"="number of subplots".

However, that fix was implemented only for vertical boxplots and not for horizontal boxplots, hence why the issue.
Also, that fix causes the labels of the shared axis to be drawn n times (where n is the number of subplots) in some cases, see below (matplotlib 3.3.2, code adapted from the one posted by the author of issue 36918)

```
import pandas as pd
import numpy as np
import matplotlib.pyplot as plt
import matplotlib

df1 = pd.DataFrame(np.random.randint(0, 100, size=(100, 4)), columns=list('ABCD'))
df2 = pd.DataFrame(np.random.randint(0, 100, size=(100, 4)), columns=list('ABCD'))

f, axs = plt.subplots(nrows=2, ncols=1, figsize=(10,7), sharex=True)
df1.boxplot(ax=axs[0], vert=True)
df2.boxplot(ax=axs[1], vert=True)

plt.show()
```

![before-3 3 2](https://user-images.githubusercontent.com/36233478/95907150-69242300-0d9b-11eb-9ec8-652744d9ad0e.png)

In this fix I used `set_major_formatter` with a `FixedFormatter` which fixes the issue and does not overdraw the labels:

![after-3 3 2](https://user-images.githubusercontent.com/36233478/95907331-a7214700-0d9b-11eb-960c-a566e90cc040.png)
