BUG: Horizontal boxplots on subplots throws ValueError
- [x] I have checked that this issue has not already been reported.

- [x] I have confirmed this bug exists on the latest version of pandas.

- [x] (optional) I have confirmed this bug exists on the master branch of pandas.

---


#### Code Sample, a copy-pastable example

```python
import pandas as pd
import numpy as np

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(1, 2, figsize=(10,7), sharey=True)
df1.boxplot(ax=axs[0], showfliers=False, vert=False)
df2.boxplot(ax=axs[1], showfliers=False, vert=False)
```

#### Problem description

Above code snippet throws `ValueError: The number of FixedLocator locations (8), usually from a call to set_ticks, does not match the number of ticklabels (4).` due to changes introduced in matplotlib 3.3.0.

It looks like fixes were implemented as part of #35393 in `boxplot.py` for `vert=True` but not for `vert=False`. Performing the `len(ticks) != len(keys)` check for `ticks = ax.get_yticks()` and the associated keys list doubling snippet fixes the issue.

Not sure if that is the right fix, but works for my use case.

#### Expected Output
Two sets of horizontal boxplot charts

#### Output of ``pd.show_versions()``

<details>

INSTALLED VERSIONS
------------------
commit           : 2a7d3326dee660824a8433ffd01065f8ac37f7d6
python           : 3.7.3.final.0
python-bits      : 64
OS               : Linux
OS-release       : 5.4.0-48-generic
Version          : #52-Ubuntu SMP Thu Sep 10 10:58:49 UTC 2020
machine          : x86_64
processor        : x86_64
byteorder        : little
LC_ALL           : None
LANG             : en_US.UTF-8
LOCALE           : en_US.UTF-8

pandas           : 1.1.2
numpy            : 1.19.2
pytz             : 2020.1
dateutil         : 2.8.1
pip              : 19.1.1
setuptools       : 41.0.1
Cython           : None
pytest           : None
hypothesis       : None
sphinx           : None
blosc            : None
feather          : None
xlsxwriter       : None
lxml.etree       : 4.5.2
html5lib         : None
pymysql          : None
psycopg2         : 2.8.6 (dt dec pq3 ext lo64)
jinja2           : 2.11.2
IPython          : 7.18.1
pandas_datareader: None
bs4              : None
bottleneck       : None
fsspec           : None
fastparquet      : None
gcsfs            : None
matplotlib       : 3.3.2
numexpr          : None
odfpy            : None
openpyxl         : None
pandas_gbq       : None
pyarrow          : None
pytables         : None
pyxlsb           : None
s3fs             : None
scipy            : 1.5.2
sqlalchemy       : 1.3.19
tables           : None
tabulate         : None
xarray           : None
xlrd             : 1.2.0
xlwt             : None
numba            : None

</details>
