isort 5 dropped support for Python 3.5

[karrtikr]

Originally posted by @PeterJCLaw in #12932 (comment)

Apologies if this isn't the right place to raise this, but as I understand it, isort 5 drops support for Python 3.5 (at least being run by Python 3.5, I don't know if it can still operate on Python 3.5 sources). Does shipping isort 5 mean that the Python extension is now also dropping support for Python 3.5 generally in the next release?
Since Python 3.5 is still the default in a variety of older but still supported Linuxes (for example: anything based on Debian Stretch, which is itself expected to continue with extended support until 2022), this feels like it would be worth clarifying.

[brettcannon]

Our policy is we only support Python versions supported upstream plus Python 2.7 (this is stated in the README. Python 3.5 reaches EOL on September 13 of this year which is less than 4 weeks away. So while this change accidentally cuts our Python 3.5 support short by two releases, if we revert this for our September release we would just be re-applying it in the October release. As such I don't think it's worth the hassle and breakage for users now moving to isort 5 thanks to us.

/cc @luabud in case she disagrees.