BLD/CI: 3.9 support

[jreback]

• CI for the 3.9 build is from source, we should update this to use numpy wheels (just became available IIUC) CI: install numpy from pip #36296 #36323
• pass tests & update whatsnew for 3.9 build so we can include in 1.1.3

cc @pandas-dev/pandas-core

[fangchenli]

We need to fix #34014. It won't compile right now.

[jbrockmendel]

AFAICT #34014 is fixed by #35928 + #34997

[wumpus]

Also #36298 is needed to build with -Werror on 3.9 -- I haven't run the pandas test suite with it, but it did fix pandas enough that my pandas-using project was happy in with 3.9 in travisci

[TomAugspurger]

@jbrockmendel how comfortable are you with backporting #34997 and #35928? IMO those probably decide whether we're likely to add support for Pytyhon 3.9 in 1.1.x or 1.2.0.

[simonjayhawkins]

1.1.x still needs to support 3.6. so need to make sure any changes to don't break 3.6 support.