Pin tox version

Towards more reproducible results with Tox.

.drone.yml

@@ -3,7 +3,7 @@ pipeline:
     image: python:${PYTHON_VERSION}-alpine${ALPINE_VERSION}
     pull: true
     commands:
-      - pip install tox
+      - pip install tox==3.10.0
       - tox
       - apk add --no-cache borgbackup
       - tox -e end-to-end

tox.ini

@@ -1,6 +1,7 @@
 [tox]
 envlist = py3
 skipsdist = True
+minversion = 3.10.0
 
 [testenv]
 usedevelop = True