Disable implicit switch-back to pure python mode.

[asvetlov]

The build fails loudly if aiohttp cannot be compiled with C Accellerators.
Use AIOHTTP_NO_EXTENSIONS=1 to explicitly disable C Extensions complication and switch to Pure-Python mode.
Note that Pure-Python mode is significantly slower than compiled one.

[asvetlov]

Should apply the same to multidict and yarl eventually.

[SomberNight]

Is there any chance this could be backported to the 3.7.x branch?
I have a project where performance is not critical and I would like to explicitly opt out of C extensions but the only released versions of aiohttp that already have this PR are the 4.0.0 alphas. :(

multidict and yarl already have an equivalent released :)

[SomberNight]

btw part of the reason I would not like to build the extensions is to make a binary of the larger project reproducible.
As aiohttp 3.7.x uses distutils to build the extension, I would need to use at least python 3.9 (to have python/cpython@0d30ae1) to make the aiohttp C extensions reproducible.

aiohttp master no longer uses distutils (since #4079) but that has not been released either :(

[webknjaz]

I don't think so, it seems like a major change. Also, the 3.7 release stream doesn't get updates, all the new things are supposed to go into the 3.8 now.

[SomberNight]

sure, 3.8.x would be great too, I just meant any branch that gets releases in the not-too-far future :)

[webknjaz]

I'm not sure sure that switching this behavior in a minor release is such a good idea.