Consider dropping support for older versions of Python

[tkw1536]

When making a PR I noticed that tests for older python versions are broken. In particular in Python2 the following test fails:

File "/home/travis/build/OpenMath/py-openmath/openmath/convert_pickle.py", line 14, in openmath.convert_pickle
Failed example:
    to_openmath('coucou')
Expected:
    OMString(string='coucou', id=None)
Got:
    OMBytes(bytes='coucou', id=None)

(see https://travis-ci.org/github/OpenMath/py-openmath/jobs/689618027#L207

Furthermore the Python 3.3 and Python 3.4 tests are failing because the python binaries can not be installed on Travis anymore.

We should consider if we want to keep supporting or testing these old versions of Python, in particular as all of them have reached end of life.

[slel]

NumPy has a "time window-based" policy for Python version support,
described in NumPy Enhancement proposal 29:

• https://numpy.org/neps/nep-0029-deprecation_policy.html

SageMath is proposing to adopt it too:

• https://trac.sagemath.org/ticket/30384