Checking plot labels works locally but not during CI
#### Code Sample, a copy-pastable example if possible

xref #31207 

If I modified the test as follows (with `import re` at the top of the file)

```python
        with tm.assert_produces_warning(None):
            fig, ax = self.plt.subplots()
            ts.plot(ax=ax)
            fig.canvas.draw()
            labels = [i.get_text() for i in ax.get_xticklabels()]
            # Extract H:M component, check first point is in correct timezone.
            # NOTE: this test could be updated once GH 31548 is fixed,
            # so that the last point is checked as well.
            assert re.findall(r"[^:]?(\d{2}:\d{2})", labels[0])[0] == "00:00"
            _check_plot_works(ts.plot)
```

then the test worked locally, but not during CI. Looking at the job details for linux py36_local, I could see that `labels` was
```
['01 00:01', ...1 00:51', ...]
```

I don't understand where that extra minute is coming from (nor can I understand why it only appears during CI).

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

<details>

[paste the output of ``pd.show_versions()`` here below this line]
INSTALLED VERSIONS
------------------
commit           : 6e2d3aeb6179d40abfb68fdcc5b064a1d6b0bc59
python           : 3.7.6.final.0
python-bits      : 64
OS               : Linux
OS-release       : 4.15.0-74-generic
machine          : x86_64
processor        : x86_64
byteorder        : little
LC_ALL           : None
LANG             : en_GB.UTF-8
LOCALE           : en_GB.UTF-8

pandas           : 0.26.0.dev0+2036.g6e2d3aeb6
numpy            : 1.17.3
pytz             : 2019.3
dateutil         : 2.8.1
pip              : 19.3.1
setuptools       : 45.0.0.post20200113
Cython           : 0.29.14
pytest           : 5.3.3
hypothesis       : 5.1.5
sphinx           : 2.3.1
blosc            : None
feather          : None
xlsxwriter       : 1.2.7
lxml.etree       : 4.4.2
html5lib         : 1.0.1
pymysql          : None
psycopg2         : None
jinja2           : 2.10.3
IPython          : 7.11.1
pandas_datareader: None
bs4              : 4.8.2
bottleneck       : 1.3.1
fastparquet      : 0.3.2
gcsfs            : None
lxml.etree       : 4.4.2
matplotlib       : 3.1.2
numexpr          : 2.7.1
odfpy            : None
openpyxl         : 3.0.1
pandas_gbq       : None
pyarrow          : 0.15.1
pytables         : None
pytest           : 5.3.3
pyxlsb           : None
s3fs             : 0.4.0
scipy            : 1.4.1
sqlalchemy       : 1.3.12
tables           : 3.6.1
tabulate         : 0.8.6
xarray           : 0.14.1
xlrd             : 1.2.0
xlwt             : 1.3.0
xlsxwriter       : 1.2.7
numba            : 0.47.0

</details>
