[Python] fix api file name & api var name

[JoeCqupt]

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.
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[JoeCqupt]

org.openapitools.codegen.languages.AbstractPythonCodegen#toApiFilename

@Override
    public String toApiFilename(String name) {
        // replace - with _ e.g. created-at => created_at
        name = name.replaceAll("-", "_");

        // e.g. PhoneNumberApi.py => phone_number_api.py
        return underscore(name + "_" + apiNameSuffix);
    }

this method has some problem

1. when apiNameSuffix is blank , api file name will end with '_'
2. apiNamePrefix param is not effect

i want modify it to :

 @Override
    public String toApiFilename(String name) {
        // e.g. PhoneNumberApi.py => phone_number_api.py
        return underscore(toApiName(name));
    }

but it is incompatible with some situation，
eg：
old: FakeClassnameTags123Api -> fake_classname_tags_123_api.py
new: FakeClassnameTags123Api -> fake_classname_tags123_api.py

[JoeCqupt]

call for review @taxpon @frol @mbohlool @cbornet @kenjones-cisco @tomplus @Jyhess @spacether

[JoeCqupt]

same to toApiFilename, toApiVarName method also need modify

[spacether]

What happens when name length is 0 with your update?

[JoeCqupt]

after update, it will call method: toApiName(name)

    public String toApiName(String name) {
        if (name.length() == 0) {
            return "DefaultApi";
        }
        return camelize(apiNamePrefix + "_" + name + "_" + apiNameSuffix);
    }

this method will get default name where name length is 0

[spacether]

This PR changes the api file names, making it a breaking change.
Existing users of autogenerated apis will have their code break if they are referencing api files whose module names no longer exist.
As is this must wait for the 6.0.0 branch.
A solution to get this merged sooner is to add an additional property command line arg which activates this new path when it is set to true, and defaults to false, keeping the current master branch file names. That would allow you to release in a minor or patch release.
Which do you prefer to do?

[JoeCqupt]

This PR changes the api file names, making it a breaking change. Existing users of autogenerated apis will have their code break if they are referencing api files whose module names no longer exist. As is this must wait for the 6.0.0 branch. A solution to get this merged sooner is to add an additional property command line arg which activates this new path when it is set to true, and defaults to false, keeping the current master branch file names. That would allow you to release in a minor or patch release. Which do you prefer to do?

@spacether i can wait for 6.0.0 branch

[wing328]

UPDATE: I've changed the target branch to 6.0.x