Try to work around Rust cryptography issue

docker/3_buildwheels/build.sh

@@ -34,6 +34,10 @@ docker run --rm -i \
     $DOCKER_IMAGE_NAME <<EOT
 set -e
 
+# Globally upgrade Pip, so that we can get a compatible version of the cryptography package.
+# See https://github.com/pyca/cryptography/issues/5771
+pip3 install --upgrade pip
+
 # Pin poetry to 1.0, as more recent version to not support nested filesystem package
 # dependencies.
 pip3 install wheel poetry==1.0

pyproject.toml

@@ -11,6 +11,7 @@ include = ["README.md", "LICENSE.txt"]
 
 [tool.poetry.dependencies]
 python = "~3.6"
+cryptography = "<3.4"
 pillar = {path = "../pillar"}
 attract = {path = "../attract"}
 flamenco = {path = "../flamenco"}