#6 Add OAuth revoke (DELETE /api/token)

[nstillman-te]

Part of #6.

Adds client.oauth.revoke() to revoke the access token (DELETE /api/token). The session must be authenticated with the token to revoke (e.g. TokenSession(access_token)). Returns 204 No Content; no response body. Requests-mock test verifies DELETE is sent with Authorization Bearer.

Checklist when adding a new endpoint

• Added new endpoint to the README.rst
• Ensured that my endpoint name does not repeat the name of the client. Wrong: client.users.get_user(), Correct: client.users.get()
• Typed the returned JSON using TypedDicts in berserk/types/ — N/A (204 No Content, no body)
• Written tests (requests-mock for DELETE and Bearer header)
• Added the endpoint to CHANGELOG.rst in the To be released section