[Python] default values escaping more appropriate

[HeikoStudt]

Fixes #15541

Default Value: Backslash value needs an escaped backslash in generated python code.
Single quotes needs escapng on multilines as well (at least if there were three in a row)

Added a small unit test.

PR checklist

• Read the contribution guidelines.
• Pull Request title clearly describes the work in the pull request and Pull Request description provides details about how to validate the work. Missing information here may result in delayed response from the community.
• Run the following to build the project and update samples:
  Commit all changed files.
• [~] In case you are adding a new generator, run the following additional script :
  Commit all changed files.
• File the PR against the correct branch: master (6.3.0) (minor release - breaking changes with fallbacks), 7.0.x (breaking changes without fallbacks)
• If your PR is targeting a particular programming language, @mention the technical committee members, so they are more likely to review the pull request.
  @spacether @krjakbrjak

[HeikoStudt]

The failed test seems to be infrastructure (NPM download failed)

[wing328]

all tests passed