remove lowest version test until next release

The next major release will depend on the next major releases of all
the other libraries, which is equivalent to devel right now.

tox.ini

@@ -1,7 +1,7 @@
 [tox]
 envlist =
     py{38,37,36,py3}
-    py38-{devel,lowest}
+    py38-devel
     style
     docs
 skip_missing_interpreters = true
@@ -10,11 +10,6 @@ skip_missing_interpreters = true
 deps =
     -r requirements/tests.txt
 
-    lowest: Werkzeug==0.15.5
-    lowest: Jinja2==2.10
-    lowest: itsdangerous==0.24
-    lowest: Click==5.1
-
     devel: https://github.com/pallets/werkzeug/archive/master.tar.gz
     devel: https://github.com/pallets/markupsafe/archive/master.tar.gz
     devel: https://github.com/pallets/jinja/archive/master.tar.gz