Add support for f-strings

[malthe]

This fixes #123.

Note that f-strings are very different from using str.format since they're actually compiled to bytecode and thus the restricted exec transformer is able to work its magic on the expression contained.

[icemac]

Thank you for your PR. I'm mostly fine with it.

Additionally to the requested changes in the code I'd request an entry in the change log referencing the issue this PR fixes.

[icemac]

I'd like these tests to be skipped on Python < 3.6 instead of having no assertion.

We already have a constant for >= 3.6:

RestrictedPython/src/RestrictedPython/_compat.py

Line 10 in 49fe489

IS_PY36_OR_GREATER = _version.major == 3 and _version.minor >= 6

The following code uses a decorator for the skipping:

RestrictedPython/tests/transformer/operators/test_arithmetic_operators.py

Lines 38 to 41 in 9d3d403

	@pytest.mark.skipif(
	not IS_PY35_OR_GREATER,
	reason="MatMult was introducted on Python 3.5")
	def test_MatMult():

[icemac]

@malthe Thank you!