Add MyPlexAccount.ping() to refresh authentication token

[JonnyWong16]

Description

Add MyPlexAccount.ping() method to ping the Plex.tv API. The /api/v2/ping endpoint is used to refresh the Plex authentication token to prevent it from expiring.

Type of change

Please delete options that are not relevant.

• New feature (non-breaking change which adds functionality)
• This change requires a documentation update

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have added or updated the docstring for new or existing methods
• I have added tests when applicable