Upgrade pip before installing other packages.

Cryptography now requires Rust. Use the newer pip to install the
pre-built wheel.

Dockerfile

@@ -6,9 +6,9 @@ RUN apt-get update && \
         python3-pip \
         python3-venv \
     && \
+    python3 -m pip install --progress-bar=off --no-cache-dir --upgrade pip && \
     python3 -m pip install --progress-bar=off --no-cache-dir --upgrade \
         docutils \
-        pip \
         pipenv \
         poetry \
         pygments \