Start v2.0 branch. Remove support for Python 2.7 & 3.4.

[tburrows13]

Changes in this PR:

• Remove most compatibility try/except blocks that allowed Py2.7 support.
• Remove compat.py and replace all places where it was used with hardcoded Py3.7 code.
• Similar with tools.is_string
• Stop Travis testing on Py2.7
• Clean up version restrictions in setup.py

I've not removed Py3.5 support because there doesn't appear to be any convincing reason to unless we decide to explicitly use a Py3.6 feature or we start hitting compatibility problems with it. There is only one difference it makes at the moment; there seems to be issues with testing it with matplotlib, but tests are easily skipped with @pytest.mark.skipif(PYTHON_VERSION == '3.5' and TRAVIS, reason="travis py35") like in test_examples.py.

[coveralls]

Coverage decreased (-0.2%) to 64.416% when pulling 64badbc on tburrows13:v2 into c901e1a on Zulko:v2.

[tburrows13]

Closes #1081.