Fix/logfmtrenderer quotes escaping

[davellas]

Summary

PR to fix escaping double quotes in LogfmtRenderer (fix https://github.com/hynek/structlog/issues/511 )

Pull Request Check List

• [x] Added tests for changed code.
  • The CI fails with less than 100% coverage.
• [x] New APIs are added to typing_examples.py.
• [x] Updated documentation for changed code.
  • [x] New functions/classes have to be added to docs/api.rst by hand.
  • [x] Changed/added classes/methods/functions have appropriate versionadded, versionchanged, or deprecated directives. Find the appropriate next version in our __init__.py file.
• [x] Documentation in .rst and .md files is written using semantic newlines.
• [x] Changes (and possible deprecations) are documented in the changelog.
• [x] Consider granting push permissions to the PR branch, so maintainers can fix minor issues themselves without pestering you.