docs: Add upgrade guide for dropping python 2.7, 3.4 & 3.5 (#570)

Author: JenniferMah

UPGRADE.md

@@ -1,8 +1,44 @@
 # Upgrade Guide
 
-_After `6.0.0` all `MINOR` and `MAJOR` version bumps will have upgrade notes 
+_`MAJOR` version bumps will have upgrade notes
 posted here._
 
+[2021-09-22] 6.x.x to 7.x.x
+---------------------------
+### Overview
+Version `7.x.x` is the first version that officially drops support for Python versions 2.7, 3.4, and 3.5.
+#### Removal of files and dependencies that were added to support Python 2.7, 3.4, and 3.5:
+- [Six](https://github.com/twilio/twilio-python/pull/560/files#diff-4d7c51b1efe9043e44439a949dfd92e5827321b34082903477fd04876edb7552L4)
+    - Removed use of `u` a fake unicode literal
+    - Removed use of `b` a fake bytes literal
+    - Removed `PY3` a boolean indicating if the code is running on Python 3
+    - `text_type` type for representing (Unicode) textual data --> `str`
+    - `iteritems` returns an iterator over dictionary’s items --> `items`
+    - `string_types` possible types for text data like basestring() in Python 2 and str in Python 3.--> `str`
+- [twilio/compat.py](https://github.com/twilio/twilio-python/pull/560/files?file-filters%5B%5D=.md&file-filters%5B%5D=.py&file-filters%5B%5D=.toml&file-filters%5B%5D=.txt&file-filters%5B%5D=.yml&file-filters%5B%5D=No+extension#diff-e327449701a8717c94e1a084cdfc7dbf334c634cddf3867058b8f991d2de52c1L1)
+    - `from twilio.compat import urlencode` --> `from urllib.parse import urlencode`
+    - `izip` --> `zip`
+- [twilio/jwt/compat.py](https://github.com/twilio/twilio-python/pull/560/files?file-filters%5B%5D=.md&file-filters%5B%5D=.py&file-filters%5B%5D=.toml&file-filters%5B%5D=.txt&file-filters%5B%5D=.yml&file-filters%5B%5D=No+extension#diff-03276a6bdd4ecdf37ab6bedf60032dd05f640e1b470e4353badc787d80ba73d5L1)
+    - Removed `compat.compare_digest`
+- [twilio/jwt/__init__.py](https://github.com/twilio/twilio-python/pull/560/files?file-filters%5B%5D=.ini&file-filters%5B%5D=.py&file-filters%5B%5D=.yml#diff-9152dd65476e69cc34a307781d5cef195070f48da5670ed0934fd34a9ac91150L12-L16)
+    - Removed import for `simplejson` and `json`
+
+#### Updated dependencies
+- [Updated PyJWT to >=2.0.0](https://github.com/twilio/twilio-python/pull/560/files#diff-4d7c51b1efe9043e44439a949dfd92e5827321b34082903477fd04876edb7552L6)
+
+### CHANGED - [Remove the ability to override the `algorithm` in `Jwt.to_jwt()`](https://github.com/twilio/twilio-python/pull/560/commits/dab158f429015e0894217d6503f55b517c27c474).
+#### Removed the ability to override the algorithm while using a jwt access token:
+```python
+// 6.x.x
+from twilio.jwt.access_token import AccessToken
+token.to_jwt(algorithm='HS512')
+```
+```python
+// 7.x.x
+from twilio.jwt.access_token import AccessToken
+token.to_jwt()
+```
+
 [2017-09-28] 6.6.x to 6.7.x
 ---------------------------