Pin pip version: cherrypick of https://projects.torsion.org/witten/borgmatic/pulls/172

.drone.yml

@@ -3,6 +3,7 @@ pipeline:
     image: python:${PYTHON_VERSION}-alpine${ALPINE_VERSION}
     pull: true
     commands:
+      - pip install --upgrade pip==19.1.1
       - pip install tox==3.10.0
       - tox
       - apk add --no-cache borgbackup

NEWS

@@ -2,7 +2,7 @@
  * Part of #125: Color borgmatic (but not Borg) output when using an interactive terminal.
  * #168: Update README with continuous integration badge.
  * Document installing borgmatic with pip install --user instead of a system Python install.
- * Get more reproducible builds by pinning the version of tox used to run tests.
+ * Get more reproducible builds by pinning the versions of pip and tox used to run tests.
 
 1.3.3
  * Add validate-borgmatic-config command, useful for validating borgmatic config generated by