Enable travis pipelines (and do some minor improvements)

[webknjaz]

What do these changes do?

• Introduce build stages support (Build Stages: Flexible and practical Continuous Delivery pipelines travis-ci/beta-features#11)
• Improve wheel build scripts (hacked during @EuroPython 2017 sprints along with @asvetlov to be simple and copy-pastable across @aio-libs)
• Do exactly one deployment to PYPI
• Install extra dependencies only where needed
• Use VM instead of docker based jobs only in deploy stage
• Move dependencies to requirements/*.txt's with use of environment markers where needed
• Add setup.py check's

Are there changes in behavior for the user?

No.

Related issue number

Closes #1926.

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.bug)
  • 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.
    • .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."

[webknjaz]

@asvetlov have fun :)

[codecov-io]

Codecov Report

Merging #2099 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #2099   +/-   ##
=======================================
  Coverage   97.08%   97.08%           
=======================================
  Files          38       38           
  Lines        7689     7689           
  Branches     1344     1344           
=======================================
  Hits         7465     7465           
  Misses        102      102           
  Partials      122      122

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