Upgrade pip to a particular version during local test and CI.

.drone.yml

@@ -3,7 +3,7 @@ pipeline:
     image: python:${PYTHON_VERSION}-alpine${ALPINE_VERSION}
     pull: true
     commands:
-      - pip install --upgrade pip==19.1.1
+      - python -m pip install --upgrade pip==19.1.1
       - pip install tox==3.10.0
       - tox
       - apk add --no-cache borgbackup

scripts/pip

@@ -0,0 +1,6 @@
+#!/usr/bin/env sh
+
+# Temporary work around for https://github.com/pypa/pip/issues/6434
+python -m pip install --upgrade pip==19.1.1
+python -m pip install --no-use-pep517 $*
+

tox.ini

@@ -8,8 +8,7 @@ usedevelop = True
 deps = -rtest_requirements.txt
 whitelist_externals = sh
 install_command =
-    # temporary work around for https://github.com/pypa/pip/issues/6434
+    sh scripts/pip {opts} {packages}
-    python -m pip install --no-use-pep517 {opts} {packages}
 commands =
     pytest
     sh scripts/black