Fix tests on Py3

[jmadler]

This change brings our test running suite green for all environments. It includes:

• Rename the PY35 constant to PY35_PLUS, which is a bit more accurate as it is valid for .e.g Py3.6.
• Bring a failed test case in-line with the behavior for the cpython interpreter.
• Incorporate @jparise's change in Skip fileno-reading test on macOS (darwin) #391