Drop support for Python 3.5

[facundobatista]

After 0.9 is out, we should stop supporting 3.5.

We want to support old Python versions, people has old systems (specially in the environments we aim CDPedia to), but we can not support everything in the past.

3.5 is in...

• Ubuntu Xenial, which loses maintenance next month (after 5 years)
• Debian old stable (Debian stable has 3.7)
• CentOS 6? don't know, but it's not maintained already

We should support 3.6; old Debians and Ubuntus have newer Python than that, but it's the default in Cent OS 7