Fix regex in Python server model code

Created by: tomghyselinck

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Description of the PR

The generated Python server model code incorrectly escapes the regular expressions. Fixes #1823 (closed)

The code in the Python client was generated correctly. Overriding DefaultCodegen::toRegularExpression seems to do the trick.

The client also overrides DefaultCodegen::addRegularExpressionDelimiter but it does not seem to be required. It is even incorrect IMHO (please correct me if I'm wrong).

I did tests on a local build with our company's product OpenAPI spec file. All regular expressions seem to be correct now.