Deprecate python 3.5

[ShadowJonathan]

Description:

Python 3.5 has reached End-Of-Life on the 13th of September, deprecating this could open up synapse to (for example):

• f-string literals (Convert all string concatenations and brace-formatting to f-string formatting #8332)
• Variable typing annotations (Type-annotate all function arguments and internal variables #8351)
• Async generators
• Async list literals (or "comprehensions")

(3.6 release notes)

[clokep]

Note that Debian Stretch comes with Python 3.5 and is in LTS support until June 2022. Does not mean we have to support Python 3.5, but is worth considering.

[callahad]

(Just learned about endoflife.date from this; thanks @ShadowJonathan!)

I did some cursory digging and it sounds like at least one supported customer may still be deploying on 3.5. Will dig further...

[ShadowJonathan]

@callahad Thanks for taking the interest, and welcome to matrix.org! 👋

[richvdh]

I'm going to close this in favour of #8782.