Improve .po IO

[akx]

• read_po: officially note that it only requires an iterable of strings, not actually a filelike (which is also an iterable of strings)
• write_po: split-refactor into generate_po() that just yields strings instead of requiring a file

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 90.99%. Comparing base (40e60a1) to head (51c741c).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #1068 +/- ## ======================================= Coverage 90.99% 90.99% ======================================= Files 26 26 Lines 4444 4444 ======================================= Hits 4044 4044 Misses 400 400 ```

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