Fixed regular expression in python client codegen that was removing any trailing chars instead of only expected ones

Created by: antssilva96

The previous regular expression was too open, including any last character in the second group. This commit fixes that, making sure we only remove leading forward slashes, trailing forward slashes or trailing \i.

Added a UT that failed prior to this change and edited an existing UT to cover the case where the regex field is not a first class citizen but instead a property inside an object (or Schema).

You can see the original issue for more details.

This commit closes 13069.

Weird things for reviewers to pay attention to:

1. In the testRegexObjects there is an assert that the property1.pattern was appended with the leading and trailing slashes (this is done in DefaultCodegen#toRegularExpression). However, if we check the generated property1.example prior to my change, it was wrong because the example generation uses the schema.pattern that does not have the leading and trailing slashes appended. I'm unsure if there is another bug somewhere... perhaps we want to use the pattern with appended slashes in the schema ?
2. The appending of / to the regular expressions in DefaultCodegen#toRegularExpression is used in DefaultCodegen#fromParameter, DefaultCodegen#fromResponse, DefaultCodegen#fromFormProperty but if the UUID is a first class citizen like in the example I provided in the issue or the file I added for the new test, it does not pass in any of these meaning the pattern does not have the slashes added. Is this the expected behavior ?

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:

  ./mvnw clean package 
  ./bin/generate-samples.sh
  ./bin/utils/export_docs_generators.sh

  Commit all changed files. This is important, as CI jobs will verify all generator outputs of your HEAD commit as it would merge with master. These must match the expectations made by your contribution. You may regenerate an individual generator by passing the relevant config(s) as an argument to the script, for example ./bin/generate-samples.sh bin/configs/java*. For Windows users, please run the script in Git BASH.
• File the PR against the correct branch: master (6.1.0) (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.