Make chardet dependency mandatory

[asvetlov]

Why not?
aiohttp should be installed from PyPI anyway.
chardet is pretty stable.
We can add extra dependency to setup.py, cleanup code a bit and drop redundant tests.

[fafhrd91]

Ok. I am fine with that

[asvetlov]

I'll do then

[ludovic-gasc]

It's a good idea.

[asvetlov]

Done in b38fdf5

[lock]

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