Update python deps for buildbot-mlir-nvidia.

[mehdi_amini]

I think Christian will need to redeploy the image, I don't think it is automatic.

[jpienaar]

Thanks!

buildbot/google/docker/buildbot-mlir-nvidia/Dockerfile

 #===-- Dockerfile --------------------------------------------------------===//
 # Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 # See https://llvm.org/LICENSE.txt for license information.
 # SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 #
 #===----------------------------------------------------------------------===//
 # Docker image used for the mlir-nvidia builder
-# 
+#
 # Environment variables configurable at runtime:
 #    BUILDBOT_PORT - server port to connect to
 #===----------------------------------------------------------------------===//
 
 # Use the image from NVIDIA as base
 FROM nvidia/cuda:10.2-base-ubuntu18.04
 
 
 # install build tools
 RUN apt-get update; \
     apt-get install -y software-properties-common apt-transport-https ca-certificates \
     clang-8 lld-8 ninja-build git wget gnupg ccache \
     python3 python3-pip python3-psutil \
     # dumb-init recommended in
     # https://hub.docker.com/r/buildbot/buildbot-worker/dockerfile
     dumb-init;\
     update-alternatives --install /usr/bin/clang clang /usr/bin/clang-8 100 ;\
     update-alternatives --install /usr/bin/clang++ clang++ /usr/bin/clang++-8 100 ;\
     update-alternatives --install /usr/bin/lld lld /usr/bin/lld-8 100
 
-# PyBind11 and Numpy are required for MLIR bindings
-RUN python3 -m pip install numpy==1.19.3 pybind11==2.6.0
+# LTS releases often bundle obsolete pip versions that cannot access newest
+# Linux binary wheels. This pinned version is not special: it was just current
+# at the time this was added. Refer to compatibility table:
+# https://github.com/pypa/manylinux
+RUN python3 -m pip install --upgrade pip==21.0.1
+
+# Refer to mlir/lib/Bindings/Python/requirements.txt. Listed explicitly here
+# and version pinned for consistency as this is a bot.
+RUN python3 -m pip install numpy==1.19.3 pybind11==2.6.0 PyYAML==5.4.1
 
 # install cuda
 # avoid popups for keyboard configurations
 RUN DEBIAN_FRONTEND=noninteractive apt-get install -y cuda
 
 # Ubuntu ships with old cmake version, install the latest one
 # from https://apt.kitware.com/
 RUN wget -O - https://apt.kitware.com/keys/kitware-archive-latest.asc 2>/dev/null | \

 # copy startup script
 COPY run.sh /home/buildbot/
 RUN chmod a+rx /home/buildbot/run.sh
 
 USER buildbot
 ENV WORKER_NAME="mlir-nvidia"
 
 # Allow the server port of this agent to be configurable during deployment.
-# This way we can connect the same image to production and integration. 
+# This way we can connect the same image to production and integration.
 # Ports:
 #   9990 - production
 #   9994 - integration
 ENV BUILDBOT_PORT="9994"
 
 CMD /home/buildbot/run.sh