Fixing the build... (drop 2.7 and 3.5 ?)

[altendky]

I submitted #548 and when the build failed I looked around and noticed that master was broken as well. First off, a little black was needed. Then an update to install-poetry.py from get-poetry.py. But, that only supports 3.6+. So... where are we going with this? I started a few changes in #549.

[hugovk]

Many projects have already dropped them, and especially when supporting them becomes a burden, sounds like a good time to drop them.

They're also little used.

Here's the pip installs for Pendulum from PyPI for May 2021:

category	percent	downloads
3.7	63.52%	2,484,047
3.6	19.32%	755,497
3.8	10.44%	408,318
3.5	2.11%	82,488
null	1.63%	63,852
3.9	1.55%	60,592
2.7	1.42%	55,472
3.10	0.00%	112
3.4	0.00%	63
Total		3,910,441

Source: pip install -U pypistats && pypistats python_minor pendulum --last-month

[Secrus]

The next version is set to support python 3.7+. Closing.