Stop testing cloudpickle against Python 3.5

[pierreglaser]

Python 3.5 reached end of life 9 months ago (https://www.python.org/downloads/release/python-3510/) - this PR removes the CI entry testing cloudpickle against Python 3.5.

[codecov]

Codecov Report

Merging #427 (386ae35) into master (a20b804) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #427   +/-   ##
=======================================
  Coverage   91.80%   91.80%           
=======================================
  Files           4        4           
  Lines         671      671           
  Branches      137      137           
=======================================
  Hits          616      616           
  Misses         34       34           
  Partials       21       21           

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[pierreglaser]

We also need to update the cloudpickle requirements in conda-forge, in the same manner as when we dropped support for Python 2 (conda-forge/cloudpickle-feedstock#36)

[ogrisel]

Could you also update:

cloudpickle/setup.py

Line 54 in 2a79b69

python_requires='>=3.5',

and delete:

cloudpickle/setup.py

Line 42 in 2a79b69

'Programming Language :: Python :: 3.5',

and add an entry in the changelog?