Attempt to upgrade version of pip used in CI.

scripts/pip

@@ -1,6 +0,0 @@
-#!/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 $*
-

scripts/run-tests

@@ -6,7 +6,7 @@
 
 set -e
 
-python -m pip install --upgrade pip==19.1.1
+python -m pip install --upgrade pip==19.3.1
 pip install tox==3.14.0
 tox
 apk add --no-cache borgbackup

tox.ini

@@ -10,8 +10,6 @@ deps = -rtest_requirements.txt
 whitelist_externals = 
     find
     sh
-install_command =
-    sh scripts/pip {opts} {packages}
 commands_pre =
     find {toxinidir} -type f -not -path '{toxinidir}/.tox/*' -path '*/__pycache__/*' -name '*.py[c|o]' -delete
 commands =