[MNT]: Make _ttconv.convert_ttf_to_ps take a binary file as output argument, not a text file #25265

Summary

convert_ttf_to_ps really outputs bytes, not str; the decoding as latin1 currently done (#25253) is meaningless if one really passed in a text file open with an ascii-incompatible encoding. Let's just make that function take a binary file as output arg instead.
See also https://github.com/matplotlib/matplotlib/pull/25253/files#r1111664168; probably this can wait for #25253 to be merged first.

Proposed fix

No response