Specify minimal required python version

[ethframe]

Which problem is this PR solving?

• Resolves #311

Short description of the changes

• Specify minimal required python version using python_requires, as suggested in linked issue. To completely resolve this issue, the current latest package (4.5.0) should be removed from PyPI

[codecov[bot]]

Codecov Report

Merging #314 (80fc2f6) into master (a30a03e) will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #314   +/-   ##
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  Coverage   95.42%   95.42%           
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  Files          25       25           
  Lines        1967     1967           
  Branches      272      272           
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  Hits         1877     1877           
  Misses         56       56           
  Partials       34       34           

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[ethframe]

current latest package (4.5.0) should be removed from PyPI

On second thought, removing package from PyPI is a bad idea, and it would be better to release a new fixed package with version 4.5.2, and then reupload 4.4.0 as 4.5.1.

[condorcet]

I think it would be better to release post-version of existing package according to PEP 440 https://www.python.org/dev/peps/pep-0440/#post-releases The changes are corresponding semantic of post-release, because it's actually not a bug fix So latest release will be 4.5.0.post1

[yurishkuro]

Why can't we just release a new version?

[ethframe]

Why can't we just release a new version?

Because pip on python 2.7 will still install the incompatible 4.5.0 if user doesn't pin specific version or there is no newer package that actually works on 2.7.