Fix MPL dt conversion (epoch2num deprecation) #34850
Fix datetime conversion after matplotlib (3.3.0 update) deprecating epoch2num.

- [ ] closes #34850
- [ ] tests added / passed
- [ ] passes `black pandas`
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

Thanks @misantroop. Seems to be some test failures. Can you check?

```
____________________ TestDateTimeConverter.test_conversion _____________________
[gw2] darwin -- Python 3.6.10 /Users/runner/miniconda3/envs/pandas-dev/bin/python

self = <pandas.tests.plotting.test_converter.TestDateTimeConverter object at 0x161aaac18>

    def test_conversion(self):
        rs = self.dtc.convert(["2012-1-1"], None, None)[0]
        xp = datetime(2012, 1, 1).toordinal()
>       assert rs == xp
E       assert 15340.0 == 734503

pandas/tests/plotting/test_converter.py:150: AssertionError
```

Can you post images of the outputs with / without your changes for matplotlib 3.3 and an earlier version? Please see an alternative circumvent that takes into account the same problem here:
https://github.com/pandas-dev/pandas/issues/34850#issuecomment-661249836 Issue fixed in MPL: https://github.com/matplotlib/matplotlib/pull/17983