Switch base methods to return Self instead of BoundLoggerBase

[aThorp96]

Summary

This Pull Request switches several BoundLoggerBase methods to annotate a return type of typing_extensions.Self instead of BoundLoggerBase. This change is consistent with the code which returns specifically self.__class__() and with the tests that ensure the type is preserved when .bind() is called.

This should not have any runtime changes, but instead fixes a typing bug which caused the following code to incorrectly fail type-checking:

from structlog import get_logger, BoundLogger

log: BoundLogger = get_logger()
log.info("This is OK")
log = log.bind() #  error: Type "BoundLoggerBase" is not assignable to declared type "BoundLogger"
log.info("This should also be okay")

Before this change:

After this change:

Pull Request Check List

It does not appear to me that any of these apply, but I am happy to make any necessary changes to the PR or do any upkeep tasks

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• [ ] Added tests for changed code.
  • The CI fails with less than 100% coverage.
• [ ] New APIs are added to our typing tests in api.py.
• [ ] Updated documentation for changed code.
  • [ ] New functions/classes have to be added to docs/api.rst by hand.
  • [ ] Changed/added classes/methods/functions have appropriate versionadded, versionchanged, or deprecated directives.
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• [ ] Documentation in .rst and .md files is written using semantic newlines.
• [ ] Changes (and possible deprecations) are documented in the changelog.
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[aThorp96]

Forgot to switch PR from my main branch to my non-main branch