Add Python 3.9 to Synapse tests

[callahad]

Synapse supports Python 3.9.

We should probably have some tests to make sure it stays that way.

Right now our pipeline yields a test matrix like this:

Twisted Trial:

	3.5	3.6	3.7	3.8	3.9
Sqlite	✅¹	✅	✅
Postgres 9.5	✅
Postgres 9.6
Postgres 10
Postgres 11			✅
Postgres 12				✅
Postgres 13

¹: Also includes an additional olddeps variant

Sytest:

	3.5	3.6	3.7	3.8	3.9
Sqlite	💧
Postgres 9.5
Postgres 9.6	💧💦
Postgres 10
Postgres 11			💦²
Postgres 12				💧
Postgres 13

💧: Monolith
💦: Multiple Workers
²: Also includes an additional with redis variant

Synapse Port DB:

	3.5	3.6	3.7	3.8	3.9
Sqlite
Postgres 9.5	✅
Postgres 9.6
Postgres 10
Postgres 11			✅
Postgres 12
Postgres 13

This raises all kinds of questions (Do we really need all of those Postgres versions? And why those specific ones, when upstream is up to 13.0?)... but more immediately, where should Python 3.9 fit in here?

Seems like there are holes for Twisted Trial runs with:

• Py 3.8 + Sqlite
• Py 3.9 + Sqlite
• Py 3.9 + Postgres ?.?

But what about Sytest?

[callahad]

Input from @matrix-org/synapse-core would be appreciated.

[clokep]

This raises all kinds of questions (Do we really need all of those Postgres versions? And why those specific ones, when upstream is up to 13.0?)... but more immediately, where should Python 3.9 fit in here?

A bit of data is that 9.5 is the earliest maintained version that still receives security updates. I also think PostgreSQL changed their versioning recently to more aggressively update the major version number. Do we have an earliest version of PostgreSQL we support? I'm not sure if we use specific features.

[erikjohnston]

We require 9.5 or later

[callahad]

Looks like I mistranscribed pg versions numbers in the tables above; fixed.

There's a hole where we're not testing anything against Postgres 10, nor the recently released 13 -- I've added those (and Python 3.9) to the tables to give a more complete picture.

[callahad]

I'm wondering if our Postgres testing strategy should just target the oldest supported (py3.5 + pg9.5) and newest supported (py3.9 + pg13) combinations. Is there much value in scattershot testing of the intermediate Postgres versions?

Similar question on the sqlite side: Should we run the Trial tests on every supported Python version, or just our oldest + newest?

[richvdh]

I think we started out testing oldest supported and newest supported, and then python 3.8 and pg12 came along so we just added more jobs to test that.

There are enough differences between python versions that I would advocate testing all of them in the range in some form: there have been instances where we end up with stuff that works on 3.5 and 3.7 but not 3.6.

[anoadragon453]

I agree with testing intermediate python versions, though not sure we need to do the same for postgres. Oldest and newest postgres sounds like a fine tradeoff for me.