Drop Python 3.5 support

[victor-torres]

Currently, the sync version of the API offers Python 3.5 support while the async version doesn't.

I believe we should ditch Python 3.5 support and focus on offering a consistent dependency set.

Complementary reasons:

• Python 3.5 is also getting too old and will be soon (if not already) out of support
• it doesn't even support the infamous f-strings
• if doesn't support all the current Type Hint features
• It shouldn't be so difficult to move a project using Python 3.5 to at least Python 3.6