Make sure OAuth1Session is using retry logic

garth.http.Client.sess uses requests.Session and has proxies, verify and adapters set. For part of the login, garth.sso uses requests_oauthlib.OAuth1Session. The proxies and verify were being set but not the adapters, which include retry (including backoff) logic.

Specifically for OAuth1Session (as subclassed by GarminOAuth1Session), this release makes setting proxies and verify more DRY and uses the same https:// adapter as the main Garth session.

Given the use of garth.sso by other projects, I want to make sure it's bulletproof and will continue to find ways to improve it.

Support 3.8

Add support for Python 3.8

PR: #7
by @yihong0618

Support serializing and deserializing tokens as strings

Add support for serializing and deserializing the OAuth tokens as strings through Client.dumps() and Client.loads()

Issue: #4
PR: #5
by @yihong0618