base: Rework Python dependency installation

This commit reworks the Python dependency installation step to:

1. Fix incorrect pip upgrade implementation -- pip cannot upgrade
   itself and must be called via Python executable. This is especially
   important because the package dependency resolver was reworked in
   the pip 20.3 and the installation step requires the reworked
   dependency resolver in order to automatically resolve conflicts.

2. Specify all requirements in single `pip install` run so that the
   package dependency resolver can resolve any conflicts among all the
   requirements.

Signed-off-by: Stephanos Ioannidis <root@stephanos.io>

Dockerfile.base

@@ -108,16 +108,13 @@ RUN wget ${WGET_ARGS} https://github.com/Kitware/CMake/releases/download/v${CMAK
  rm -f ./cmake-${CMAKE_VERSION}-Linux-${HOSTTYPE}.sh
 
 # Install Python dependencies
-RUN pip3 install wheel pip -U &&\
- pip3 install -r https://raw.githubusercontent.com/zephyrproject-rtos/zephyr/master/scripts/requirements.txt && \
- pip3 install -r https://raw.githubusercontent.com/zephyrproject-rtos/mcuboot/master/scripts/requirements.txt && \
- pip3 install west &&\
- pip3 install sh &&\
- pip3 install awscli PyGithub junitparser pylint \
- statistics numpy \
- imgtool \
- protobuf \
- GitPython
+RUN python3 -m pip install -U pip && \
+ pip3 install -U wheel setuptools && \
+ pip3 install \
+ -r https://raw.githubusercontent.com/zephyrproject-rtos/zephyr/master/scripts/requirements.txt \
+ -r https://raw.githubusercontent.com/zephyrproject-rtos/mcuboot/master/scripts/requirements.txt \
+ awscli GitPython imgtool junitparser numpy protobuf PyGithub \
+ pylint sh statistics west
 
 # Clean up stale packages
 RUN apt-get clean -y && \