Issue 166 wrong error raised by session

[pvk-developer]

• Fixed error that when no tunables are left it kept trying to propose which was leading to a ValueError raise.
• Added BTBSession to btb so you can now import it directly from btb import BTBSession.
• Cap sphinx version below 2.4 as with a new release the docs are filing because of the m2r plugin that we use for .md files until a fix is released.

Resolve #166

[csala]

Codecov Report

Merging #172 into master will increase coverage by 0.00%. The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #172   +/-   ##
=======================================
  Coverage   96.35%   96.36%           
=======================================
  Files          39       39           
  Lines        1015     1017    +2     
=======================================
+ Hits          978      980    +2     
  Misses         37       37           

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[codecov-io]

Codecov Report

Merging #172 into master will increase coverage by <.01%. The diff coverage is 85.71%.

@@            Coverage Diff             @@
##           master     #172      +/-   ##
==========================================
+ Coverage   96.35%   96.36%   +<.01%     
==========================================
  Files          39       39              
  Lines        1015     1017       +2     
==========================================
+ Hits          978      980       +2     
  Misses         37       37

Impacted Files	Coverage Δ
btb/__init__.py	100% <100%> (ø)	:arrow_up:
btb/session.py	95.68% <80%> (ø)	:arrow_up:

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Legend - Click here to learn more Δ = absolute <relative> (impact), ø = not affected, ? = missing data Powered by Codecov. Last update 622736f...c0d5193. Read the comment docs.