tests/sitecustomize.py

rust: simplify pattern file parsing

Previously we kept the pattern syntax as a &[u8] until the last
possible moment, which meant we had to handle potential errors when
parsing that &[u8].  But such errors could never actually occur, given
the structure of the code.

Now we parse it eagerly (in two places) and pass it around as a
PatternSyntax, so we can delete some error handling code.

parse_one_pattern is in particular useful for parsing patterns passed
on the command line, as we'll support later in this series.

import os

if os.environ.get('COVERAGE_PROCESS_START'):
    try:
        import coverage
        import uuid

        covpath = os.path.join(
            os.environ['COVERAGE_DIR'], 'cov.%s' % uuid.uuid1()
        )
        cov = coverage.coverage(data_file=covpath, auto_data=True)
        cov._warn_no_data = False
        cov._warn_unimported_source = False
        cov.start()
    except ImportError:
        pass