Set an explicit suffix on temporary filenames for toots

[larsks]

When you write a toot using toot post -e, the editor opens a temporary file named something like /tmp/tmp1234567. Because of the generic filename it's not possible to apply any automatic settings (syntax, wrapping, spell checking, etc) to the file.

Include an explicit prefix or suffix in the name so that toots are identifiable for the purpose of customizing editor settings.

E.g.:

with tempfile.NamedTemporaryFile(suffix='.toot') as fd:
  ...