Strip out Python 2 from Iris src code

[lbdreyer]

Iris 3 will be a Python 3 only release so we need to remove all Python 2 support.

This will include removing all the six code and any other remaining Python 2 handling.

Note, we have already dropped testing with Python 2 (see #3446)

Note, this could be split into multiple smaller issues.

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Acceptance Criteria:

• Remove all six code - @trexfeathers Remove six #3497
• Remove unnecessary __future__ imports & address iris.tests.test_coding_standards.TestFutureImports - @stephenworsley Removed __future__ imports #3505
• Remove unnecessary super arguments - @stephenworsley Remove unnecessary super arguments #3506
• Remove/replace all Python2 specific code @stephenworsley Remove python 2 specific code #3507
• Address all code that deals with Python3 as optionally supported @stephenworsley Remove python 2 specific code #3507
• Remove Python2 class references to object, see here for an example @stephenworsley Simplify class definitions, remove (object) #3511
• Merge feature branch py2_strip to master
• Delete feature branch py2_strip

[bjlittle]

Remove the iris.tests.test_coding_standards.TestFutureImports class, see here.

[bjlittle]

Also, grep through the code base and inspect hits for:

• grep -ri "python3" and grep -ri "python 3"
• grep -ri "python2" and grep -ri "python 2"

[stephenworsley]

For Python 3 relevent __future__ imports, see https://docs.python.org/3/library/__future__.html

[stephenworsley]

Since it is conceivable that we might introduce __future__ imports at a later date, it might be worthwhile skipping this test, removing Python 2/3 specific references, and leaving its skeleton for future reference.

[trexfeathers]

An example of some of the 'Python 2 specific code' we might be looking for:

        with self.assertRaisesRegex(TypeError,
                                    # exactly == Py2, positional == Py3
                                    'takes (exactly )?1 (positional )?'
                                    'argument '):