Add support for python 3.7/3.8

[pva]

Currently https://pypi.org/project/influxdb/ page reports python 3.6 as a maximum supported version. Please, bump at least for 3.7 as written in read.me or even 3.8. TIA.

[russorat]

@pva thanks for the issue! is this something you might be willing to help with?

[lafrech]

3.7 is tested already.

What could be done:

• Test against 3.8 as well (I'd just test 2.7, 3.5 and 3.8, no need to also test 3.6 and 3.7 IMHO)
• Update README.
• Add 3.7 and 3.8 to classifiers in setup.py
• Add python_requires directive to setup.py.