Support python 3.9

[maximlt]

(I did not see #26 before opening this PR, feel free to close mine!)

I attempted to install tranquilizer with pip in a Python 3.9 environment and ended up with a few horrific error messages and pip installing the version 0.0.5 😄

I've bumped the pins to support Python 3.9 and ran the test suite successfully in a conda env with the packages pulled from defaults. Tried Python 3.10 too but all the dependencies are not yet available on defaults (they must be on conda-forge thought, and pypi). Out of curiosity, what's the reason for pinning the upper Python version supported?

This PR also contains a few changes that remove warnings emitted during the test suite execution, and make a few minor changes to the README.

[codecov-commenter]

Codecov Report

Merging #39 (eb488ea) into master (9b4a738) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master      #39   +/-   ##
=======================================
  Coverage   96.30%   96.30%           
=======================================
  Files           7        7           
  Lines         325      325           
=======================================
  Hits          313      313           
  Misses         12       12           

Impacted Files	Coverage Δ
tranquilizer/decorator.py	100.00% <100.00%> (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 9b4a738...eb488ea. Read the comment docs.

[AlbertDeFusco]

Thanks! The upper version pins come from an abundance of caution. There were some breaking changes in the last few minor python versions.

Anyway, I'll appove and merge this. The next release will drop Py 3.5 so don't worry that the tests fail.