Declare file encoding, as per PEP-263 given our use of UTF-8

Python 2.7 uses 7-bit ASCII [1], whereas Python 3 uses Unicode [2].
To allow UTF-8 in Python 2.7 program text, an explicit declaration is
necessary [3].

[1] https://docs.python.org/2.7/reference/lexical_analysis.html
[2] https://docs.python.org/3/reference/lexical_analysis.html
[3] http://legacy.python.org/dev/peps/pep-0263/