build binaries for python 3.8

[samuelcolvin]

Change Summary

• Include binaries for python 3.8 in pypi
• add manylinux2010 binaries (by using the latest version of cibuildwheel)
• run tests on production binaries in cibuildwheel
• improve travis setup to run fewer tests and thereby finish faster

Checklist

• Unit tests for the changes exist
• Tests pass on CI and coverage remains at 100%
• Documentation reflects the changes where applicable
• changes/<pull request or issue id>-<github username>.md file added describing change
  (see changes/README.md for details)

[codecov]

Codecov Report

Merging #994 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff          @@
##           master   #994   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files          19     19           
  Lines        3287   3287           
  Branches      650    650           
=====================================
  Hits         3287   3287

Impacted Files	Coverage Δ
pydantic/version.py	100% <100%> (ø)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 677537d...49f772b. Read the comment docs.