Modify stdin retriever to detect coding pragma

When we read in a file on Python 3, we use the tokenize module to detect
the coding pragma at the top of the file. We then use that to decode the
rest of the file. However, when we were receiving stdin, we would not do
that.

This updates ``stdin_get_value`` in a backwards compatible way to check
for that coding pragma and then fall back to UTF-8 if necessary.

Closes #306