Update stripe API version to 2020-03-02

[dav1do]

• Add 2020-03-02 and 2020-08-27 API versions to the enum
• Set API header 2020-03-02 on the client. This is a breaking change but 0.13 is supposed to target 2020-03-02 according to the readme.

Questions:

• Does openAPI spec regen need to happen?
• Since Stripe uses the version of your account if you don't pass the header, it seems nice to leave off/add a new_with_version constructor, however there are probably compatibility issues with the generated types in that case, so maybe the library can only support one version at a time.