Drop Python 3.5 support

[TomAugspurger]

As discussed in #27492.

The 0.25.x branch should be stable for the next release, so we can start on this at any time.

[topper-123]

I'd be positive to deprecate python 3.5. It's old and 3.6 brings a lot of changes, so make sense to not support 3.5 in a long-term stable version.

An idea could be to make a dual release 0.26 and 1.0. They would have identical code bases, but 0.26 would support py3.5 and 1.0 would support py3.6, so it would only be a minor chang in setup.py.

[TomAugspurger]

Why do you think we need a deprecation instead of just dropping it?

[topper-123]

Sorry, I miswrote that, i meant drop.

[TomAugspurger]

Gotcha, thanks.

FTR, https://numpy.org/neps/nep-0029-deprecation_policy.html has September 2019 as the cutoff for 3.5.

[WillAyd]

@pandas-dev/pandas-core do we have thoughts on this? The only thing I can think of that warrants a 0.25.3 release right now might be the fix in #29173

If there isn't an appetite for that release I think we should drop 35 from CI now

[jreback]

+1 in dripping 3.5 for 1.0

this wouldn’t affect 0.25.x

[WillAyd]

I’m talking specifically about dropping from CI. I’m leaning towards maybe cutting a 25.3 for the regression fix and dropping 35 after that

[jreback]

whether we cut 0.25.3 is irrelevant
we have no reason to change the 0.25.x CI

[WillAyd]

Ah OK. So OK to drop from CI on master and if we want a 0.25.3 can target the 0.25.x branch directly?

[jreback]

right as usual would merge a patch to master and back port to 0.25.x

[TomAugspurger]

+1 for dropping from master.

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[gfyoung]

+1 from me as well for master