build: Install wheel instead of sdist into tox environments.

This avoids building the package separately for each environment.

Author: seifertm

tox.ini

@@ -6,6 +6,8 @@ passenv =
     CI
 
 [testenv]
+package = wheel
+wheel_build_env = .pkg
 extras = testing
 constraints = dependencies/default/constraints.txt
 deps = -r dependencies/default/requirements.txt