Add Python 3.8 to test matrix

[jdufresne]

Python 3.8 was released on October 14th, 2019.

[codecov-io]

Codecov Report

Merging #43 into master will increase coverage by 1.2%.
The diff coverage is n/a.

@@            Coverage Diff            @@
##           master      #43     +/-   ##
=========================================
+ Coverage   87.25%   88.46%   +1.2%     
=========================================
  Files           4        4             
  Lines         204      208      +4     
  Branches       28       29      +1     
=========================================
+ Hits          178      184      +6     
+ Misses         16       15      -1     
+ Partials       10        9      -1

Impacted Files	Coverage Δ
src/tblib/decorators.py	100% <0%> (ø)	⬆️
src/tblib/__init__.py	91.33% <0%> (+1.65%)	⬆️

---

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 e928392...780c22a. Read the comment docs.

[ionelmc]

I guess I'll fix this by bringing up various fixes from pylibrary (also fixes the cov reporting issues).