No need for python-dev in the base image

Now that #38 is done, no compiled extension modules -> no python-dev

dockerfile

@@ -10,6 +10,6 @@ RUN apt-get update --quiet && \
     apt-get install --assume-yes --quiet --no-install-recommends \
         git cmake make gcc g++ gfortran \
         pocl-opencl-icd ocl-icd-opencl-dev opencl-clhpp-headers \
-        python3-pyopencl python3-dev \
+        python3-pyopencl \
         python3-scipy tox \
     && apt-get clean