[BUG] [Python] Support timezone in datetime

Created by: jirikuncar

Bug Report Checklist

Have you provided a full/minimal spec to reproduce the issue?
Have you validated the input using an OpenAPI validator (example)?
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Description

The parser fails on date time with timezone information.

openapi-generator version

master

https://github.com/OpenAPITools/openapi-generator/blob/master/modules/openapi-generator/src/main/java/org/openapitools/codegen/languages/PythonClientExperimentalCodegen.java#L215-L219

OpenAPI declaration file content or url

type: string
example: '2020-02-02T02:02:02.00000Z'
format: date-time
# or
type: string
example: '2020-02-02T02:02:02.00000+02:00'
format: date-time

Command line used for generation

STDERR:
Exception in thread "main" java.lang.RuntimeException: Could not process model 'XXX_time'.Please make sure that your schema is correct!
        at org.openapitools.codegen.DefaultGenerator.generateModels(DefaultGenerator.java:496)
        at org.openapitools.codegen.DefaultGenerator.generate(DefaultGenerator.java:1015)
        at org.openapitools.codegen.cmd.Generate.execute(Generate.java:431)
        at org.openapitools.codegen.cmd.OpenApiGeneratorCommand.run(OpenApiGeneratorCommand.java:32)
        at org.openapitools.codegen.OpenAPIGenerator.main(OpenAPIGenerator.java:61)
Caused by: java.lang.ClassCastException: java.time.OffsetDateTime cannot be cast to java.util.Date
        at org.openapitools.codegen.languages.PythonClientExperimentalCodegen.toDefaultValue(PythonClientExperimentalCodegen.java:238)
        at org.openapitools.codegen.DefaultCodegen.fromProperty(DefaultCodegen.java:2898)
        at org.openapitools.codegen.languages.PythonClientExperimentalCodegen.fromProperty(PythonClientExperimentalCodegen.java:431)
        at org.openapitools.codegen.DefaultCodegen.addVars(DefaultCodegen.java:4601)
        at org.openapitools.codegen.DefaultCodegen.addVars(DefaultCodegen.java:4549)
        at org.openapitools.codegen.DefaultCodegen.fromModel(DefaultCodegen.java:2402)
        at org.openapitools.codegen.languages.PythonClientExperimentalCodegen.fromModel(PythonClientExperimentalCodegen.java:802)
        at org.openapitools.codegen.DefaultGenerator.processModels(DefaultGenerator.java:1299)
        at org.openapitools.codegen.DefaultGenerator.generateModels(DefaultGenerator.java:491)
        ... 4 more

Steps to reproduce

Related issues/PRs

Suggest a fix

Parse timezone from given string and use UTC as default.