[Cython] PEPs 263 and 3120

[Stefan Behnel]

Hi,

Stefan Behnel wrote:
> BTW, I also noticed that some places in the code incorrectly assume that you
> can write a filename into a C file comment or docstring without further
> treatment.

I fixed that also. File names get encoded to ASCII when written into the C
source and non-ASCII characters get replaced. The docstring embedding of the
original file position should also work now.

While I was at it, I also fixed the code that copies Cython source code into
the C file comments. The source is now correctly read as unicode and then
encoded to plain ASCII before using it. I wasn't sure if it helps to
backslash-escape non-ASCII characters here, so I left that out for now. It's
only a helper for human readers anyway.

Stefan