Specify version of mpi4py

Latest doesn't seem to want to build in this environment.

Author: trevilo

docker/test/Dockerfile

@@ -151,7 +151,7 @@ RUN yum -y install gsl-gnu9-ohpc
 RUN pip3 install "pybind11[global]"
 
 RUN . /etc/profile.d/lmod.sh \
-    && pip3 install mpi4py
+    && pip3 install mpi4py==3.1.5
 
 # Register new libs installed into /usr/local/lib with linker
 RUN echo "/usr/local/lib" > /etc/ld.so.conf.d/class.conf