Pin pyshtools to 4.9.1 in multires generator Dockerfile.

Latest version of pyshtools uses Meson for building and does not build
in 20.04.

utils/multires/Dockerfile

@@ -7,7 +7,7 @@ ENV DEBIAN_FRONTEND noninteractive
 RUN apt-get update && apt-get install -y --no-install-recommends \
     python3 python3-dev python3-numpy python3-pip python3-pil hugin-tools \
  && rm -rf /var/lib/apt/lists/*
-RUN pip3 install pyshtools
+RUN pip3 install pyshtools==4.9.1
 
 ADD generate.py /generate.py
 ENTRYPOINT ["python3", "/generate.py"]