unclear why python-openssl fails the build

but it does

```
Picking 'pyopenssl' as source package instead of 'python-openssl'

[91mE: Unable to find a source package for python-openssl
[0m
```

Dockerfile

@@ -38,8 +38,8 @@ RUN apt-get --quiet update && apt-get --quiet install -y \
 # magic-wormhole depends on these and pip wants to build them both from
 # source.
 RUN apt-get --quiet update && apt-get --quiet build-dep -y \
+    pyopenssl \
     python-nacl \
-    python-openssl \
 && rm -rf /var/lib/apt/lists/*
 
 # Create a virtualenv into which to install magicwormhole in to.