Replace toml with tomli

[mkniewallner]

Resolves #828

Replace toml with tomli for parsing bandit options defined in pyproject.toml.

The reason for requiring at least 1.1.0 is because support for text files has been deprecated in 1.2.0 then removed in 2.0.0, and support for binary files has only been added in 1.1.0, so this ensures that users have 1.1.0+, to reliably be able to read binary files.

As tomli only reads binary files, it is required to open the file using rb flags, which also impacts the format passed to safe_load for YAML files, but PyYAML accepts both binary and text files, so this still works as expected, though this may slow down a bit parsing time.

[mkniewallner]

As python/cpython#31498 has been merged, I also added d40fed8 to only require tomli on Python < 3.11, and updated the import so that tomli is imported as tomllib, to reflect the fact that this will be the main way to import the parser.

Note that tomllib will only be in Python 3.11 in the next pre-release, 3.11.0a7, so if we prefer to wait for that, or maybe an RC, before considering it, I would be happy to drop this second commit and make a follow up PR later on.

[ericwb]

LGTM