Treat black's NothingChanged as a successful no-op

[Eljees]

Closes #427.

The problem

black.format_file_contents reports "this is already formatted" by raising
NothingChanged rather than by returning the content unchanged.
BlackFormatter.format does not catch it, so as soon as an autofix produces
content that black has nothing to say about, the exception escapes
format_module.

fixit_bytes wraps the whole pass in a broad except Exception, so the
outcome is worse than a lost fix: the run yields a Result carrying
black.report.NothingChanged as a lint error, and the fix is dropped. That
matches the traceback in the report.

UfmtFormatter is not affected, because ufmt_bytes handles this internally.
That is why the issue only appears with formatter = "black".

The change

Catch NothingChanged and return the module bytes, which is what black would
have returned had it not signalled through an exception.

Tests

A new fixit.tests.format module. Without the change the already-formatted
case reproduces the reported failure:

ERROR: test_black_already_formatted (content=b'x = 1\n')
  File "src/fixit/format.py", line 51, in format
    content = black.format_file_contents(
  File "src/black/__init__.py", line 1117, in format_file_contents
black.report.NothingChanged

The same test also covers content that black does rewrite, so the change
cannot be satisfied by skipping formatting altogether, plus the no-formatter
path for completeness.

The suite goes from 389 to 391 tests, all passing. flake8 and ufmt check
on the touched files are clean.