Compression errors with `plaintext` on `windows`

[griff-rees]

There are compression errors for windows when testing the plaintext.PlaintextFixture class that are not reproduced in linux/darwin. Currently the docstrings for these processes skip windows. The methods affected include

• .plaintext_paths_to_dicts()
• .PlaintextFixture.export_to_json_fixtures()

There are some tests to address aspects of this in tests/test_plaintext.py