Added check install idna-ssl or not on Python3.7

[hellysmile]

What do these changes do?

idna_ssl is not installed on Python3.7 as dependency

otherwise on Python3.7 will be

idna-ssl requires Python '<3.7.0' but the running Python is 3.7.0

[codecov-io]

Codecov Report

Merging #2703 into 2.3 will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##              2.3    #2703   +/-   ##
=======================================
  Coverage   97.58%   97.58%           
=======================================
  Files          35       35           
  Lines        7439     7439           
  Branches     1295     1295           
=======================================
  Hits         7259     7259           
  Misses         71       71           
  Partials      109      109

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[webknjaz]

Same could be done with extras+markers. Smth like:

extra_requires = {
    ':python_version<"3.7"': ['idna-ssl>=1.0.0']
}

[asvetlov]

Yep, maybe your proposal is more elegant -- but I don't care too much

[webknjaz]

it's just a note for future if I ever get to doing #2397

[lock]

This thread has been automatically locked since there has not been any recent activity after it was closed. Please open a [new issue] for related bugs.
If you feel like there's important points made in this discussion, please include those exceprts into that [new issue].
[new issue]: https://github.com/aio-libs/aiohttp/issues/new