[MRG] PyPI fixes

[luizirber]

It's been some time since we uploaded releases to PyPI, and things changed:

• Warehouse was deployed, so it supports Markdown in the long_description field. I decided to put the README.md file there (which makes GitHub and PyPI look the same)
• Markdown support requires setuptools>=38.6.0, which is a fairly recent version. I bumped the version in setup.py, so this would probably also fix what version of setuptools do we require? #173
• Update README to point to pre-releases instead of installing from master
• Update DBs in docs (for 2.0.0a8)

Checklist

• Is it mergeable?
• make test Did it pass the tests?
• make coverage Is the new code covered?
• Did it change the command-line interface? Only additions are allowed
  without a major version increment. Changing file formats also requires a
  major version number increment.
• Was a spellchecker run on the source code and documentation after
  changes were made?

[codecov-io]

Codecov Report

Merging #504 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #504   +/-   ##
=======================================
  Coverage   90.69%   90.69%           
=======================================
  Files          33       33           
  Lines        4999     4999           
  Branches       36       36           
=======================================
  Hits         4534     4534           
  Misses        464      464           
  Partials        1        1

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[luizirber]

Example of rendered description:
https://test.pypi.org/project/sourmash/2.0.0a8/

[luizirber]

ready for review @ctb @taylorreiter @brooksph @bluegenes