Include relevant packages for pip install

[neerajprad]

This fixes the issue of pip install . doing an empty package install (doesn't contain any modules/files), and therefore doesn't work when we are outside the repository. cc. @martinjankowiak

• Adds pandas requirement and changes the minimum version to python 3.5 because type annotations would throw an error otherwise.
• Other modules like models or tasks can be exposed by adding the __init__.py file later, if desired.

[codecov[bot]]

Codecov Report

Merging #5 into master will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##           master       #5   +/-   ##
=======================================
  Coverage   98.74%   98.74%           
=======================================
  Files          14       14           
  Lines         798      798           
=======================================
  Hits          788      788           
  Misses         10       10

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