tox: Remove entries from default envlist (#1578)

lowest-supported-dev requires running with python 3.6, which few people
will be running locally.

hypothesis-slow is being removed because fuzz testing
shouldn't be a gate for new features.

LP: #1980854

tox.ini

@@ -1,8 +1,6 @@
 [tox]
 envlist =
     py3,
-    lowest-supported-dev,
-    hypothesis-slow,
     black,
     flake8,
     isort,