Add support for PyFile_FromFile

[javaprog]

In Python 2 there is only PyFile_FromFile, which accepts a file pointer, whereas in Python 3 there is only PyFile_FromFd, which accepts a file descriptor. To avoid changing legacy code and introducing special cases for both Python versions, I would like to suggest an implementation of PyFile_FromFile for Python 3 that simply forwards to the existing function: with the help of fileno() the file descriptor for a file pointer can be obtained.

[encukou]

To anyone looking for hints: In #45, implementing PyFile_FromFile in terms of PyFile_FromFd and fileno didn't work because it leaked FILE* structures.

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Unfortunately, "file objects" in Python 3 are quite different than in Python 2.
As the py3 docs say,

The [PyFile_* functions] are convenience C wrappers over these new APIs, and meant mostly for internal error reporting in the interpreter; third-party code is advised to access the io APIs instead.

Given that io is available on Python 2.6+, I'd rather encourage using that. It is what PyFile_FromFd does.