Reintroduce python3 in the Dockerfile

[mvalkon]

This PR reintroduces the changes made in previous PR's for moving the Dockerfile and other files to python3. The breaking changes that were discovered also seem to happen with previous images with python2. More investigation is needed.

There is no backwards compatibility for python versions here, in case such is needed a more sophisticated solution is needed.

References #80

[mvalkon]

@scherniavsky with these changes I can run my test case which is in python3.

[codecov-io]

Codecov Report

Merging #94 into master will increase coverage by 0.01%. The diff coverage is 100%.

@@            Coverage Diff            @@
##           master     #94      +/-   ##
=========================================
+ Coverage   91.79%   91.8%   +0.01%     
=========================================
  Files           7       7              
  Lines         524     525       +1     
=========================================
+ Hits          481     482       +1     
  Misses         43      43

Impacted Files	Coverage Δ
src/app.py	87.38% <100%> (+0.05%)	:arrow_up:
src/tests/test_bootstrap.py	99.13% <100%> (ø)	:arrow_up:

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[scherniavsky]

:+1:

[scherniavsky]

:+1:

[bmaher]

👍

[scherniavsky]

:+1:

[bmaher]

👍

[scherniavsky]

:+1:

[hjacobs]

:+1:

[mvalkon]

thanks @scherniavsky @bmaher @hjacobs 🎉