Office365-rest-python-client: Proposal: drop support to python 2.7

@domdinicola might be a good idea, thanks! Although would be interested to get familiar with runtime usage statistics.

p.s. and might be slow with response since on vacation until the end of the month

based on this https://en.wikipedia.org/wiki/History_of_Python we should be able to get rid of those since are not officially supported anymore.
enjoy your vacation!

Here's the pip installs for Office365-REST-Python-Client from PyPI for June 2020:

| category | percent | downloads |
|----------|--------:|----------:|
| 3.7 | 78.80% | 32,146 |
| 3.6 | 12.61% | 5,144 |
| 3.5 | 5.80% | 2,367 |
| 3.8 | 1.80% | 736 |
| 2.7 | 0.58% | 235 |
| null | 0.39% | 161 |
| 3.4 | 0.01% | 6 |
| Total | | 40,795 |

Source: pip install -U pypistats && pypistats python_minor Office365-REST-Python-Client --last-month

Since between 3.6 and 3.7 we reach more than 90%, I think we can drop 3.5 and 2.7 (3.4 is already not supported)
https://github.com/vgrem/Office365-REST-Python-Client/pull/233