A simpler, more limited fix to issue 4111.

Just change a few stray invocations of iteritems() to items(), in
order to be compatible with Python3 and consistent with previous
changes for supporting a Python2/3 unified source.

	modified:   sympy/core/compatibility.py
	modified:   sympy/core/tests/test_compatibility.py
	modified:   sympy/functions/elementary/trigonometric.py
	modified:   sympy/polys/polycontext.py
Change docstring to inline comment

This is in response to comment from smichr

	modified:   sympy/core/tests/test_compatibility.py
Merge pull request #2645 from rathmann/4111_iteritems_small

A simpler, more limited fix to issue 4111.