Fix documentation build configuration file for Windows

[Harmon758]

What does this change do?

Fixes documentation builds for Windows

Are there changes in behavior for the user?

No
Python <2.6 support is dropped for building the documentation
Not sure why codecs.open was used, but Python 2.6+ support is maintained with io.open

Related issue number

Fixes #3147

Checklist

• I think the code is well written
• Unit tests for the changes exist
• Documentation reflects the changes
• If you provide code modification, please add yourself to CONTRIBUTORS.txt
  • The format is <Name> <Surname>.
  • Please keep alphabetical order, the file is sorted by names.
• Add a new news fragment into the CHANGES folder
  • name it <issue_id>.<type> for example (588.bugfix)
  • if you don't have an issue_id change it to the pr id after creating the pr
  • ensure type is one of the following:
    • .feature: Signifying a new feature.
    • .bugfix: Signifying a bug fix.
    • .doc: Signifying a documentation improvement.
    • .removal: Signifying a deprecation or removal of public API.
    • .misc: A ticket has been closed, but it is not of interest to users.
  • Make sure to use full sentences with correct case and punctuation, for example: "Fix issue with non-ascii contents in doctest text files."

[codecov-io]

Codecov Report

Merging #3148 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #3148   +/-   ##
=======================================
  Coverage   98.09%   98.09%           
=======================================
  Files          43       43           
  Lines        7833     7833           
  Branches     1351     1351           
=======================================
  Hits         7684     7684           
  Misses         57       57           
  Partials       92       92

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[asvetlov]

I don't follow why do you need to build docs on Python 2.6?
aiohttp is intended to build by Python 3 only

[Harmon758]

I don't need to build them on Python 2, but I don't see any other reason codecs.open would be used instead of just open.
So that support for Python 2.6+ is maintained with io.open instead of dropped here, while still resolving the issue.

[asvetlov]

I don't care but if the change helps you -- why not?

[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