Fix dataclass issues for Python 3.11

[rharish101]

This PR adds Python 3.11 support.

Closes #88.

Description

Transformer doesn't work in Python 3.11, due to breaking changes in dataclasses (python/cpython#99401). Namely, the default values for dataclass fields must be hashable. In the Transformer codebase, MappingProxyType is used, which isn't hashable. This PR fixes those issues, by using default_factory to get around this issue.

Types of Changes

What types of changes does your code introduce? Keep the ones that apply:

• Bug fix (non-breaking change which fixes an issue)

Review

Reviewers' checklist:

• If this PR implements new flows or changes existing ones, are there
  good tests for these flows?
  If this PR rather removes flows, are the obsolete tests removed as well?
• Is the documentation still up-to-date and exhaustive? This covers both
  technical (in source files) and functional (under docs/) documentation.
• Is the changelog updated?
• Does the new version number correspond to the actual changes from this PR?
  In doubt, refer to https://semver.org.

After this PR

Any follow-up action necessary?

[zincr]

🤖 zincr found 1 problem , 0 warnings

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[mihaiturcu]

@thilp can we get this approved?
Thanks