When providing a file to the compile function, the PyObject_AsFileDescriptor
function is used, which is safe to use on any object type: if the object is
not a file, -1 is returned.

However, this value was passed straight to a call to dup() without checking
for its validity. This isn't much of an issue on unix where the dup call will
simply fail as well and return -1, but it is very much an issue on windows
where passing an invalid fd to dup will invoke the invalid parameter handler,
which isn't set, so it will simply terminate the program.

In other words, passing anything but a file to the file parameter
of the compile function will make the program crash on Windows.

The type of the key for the externals dict in both the compile and the
match function was not checked. Providing a dict with non string keys
leads to a segfault.

…ated.

The tests are crashing in PyPy with segfaults. It's unclear why this happens, as it is happening even with version 4.5.1, for which PyPy wheels were correctly crated and tested in the past.