Switch to a different ppa for python36.

Looks like https://launchpad.net/~jonathonf is getting slowly removed.

Fix "The repository 'http://ppa.launchpad.net/jonathonf/python-3.6/ubuntu xenial Release' does not have a Release file."

PiperOrigin-RevId: 286273233
Change-Id: I5fd9b896f03f748fea440d5ab4077d996ebe2afd

tensorflow/tools/ci_build/Dockerfile.custom_op_ubuntu_16

@@ -53,7 +53,7 @@ env GOROOT=/usr/local/go
 env PATH=$GOROOT/bin:$PATH
 
 # Install python 3.6.
-RUN add-apt-repository ppa:jonathonf/python-3.6 && \
+RUN add-apt-repository ppa:deadsnakes/ppa && \
     apt-get update && apt-get install -y \
     python3.6 python3.6-dev python3-pip python3.6-venv && \
     rm -rf /var/lib/apt/lists/* && \

tensorflow/tools/ci_build/Dockerfile.custom_op_ubuntu_16_cuda10.0

@@ -53,7 +53,7 @@ RUN /install/install_buildifier.sh
 ENV TF_NEED_CUDA=1
 
 # Install python 3.6.
-RUN add-apt-repository ppa:jonathonf/python-3.6 && \
+RUN add-apt-repository ppa:deadsnakes/ppa && \
     apt-get update && apt-get install -y \
     python3.6 python3.6-dev python3-pip python3.6-venv && \
     rm -rf /var/lib/apt/lists/* && \

tensorflow/tools/ci_build/Dockerfile.custom_op_ubuntu_16_cuda10.1

@@ -55,7 +55,7 @@ RUN /install/install_buildifier.sh
 ENV TF_NEED_CUDA=1
 
 # Install python 3.6.
-RUN add-apt-repository ppa:jonathonf/python-3.6 && \
+RUN add-apt-repository ppa:deadsnakes/ppa && \
     apt-get update && apt-get install -y \
     python3.6 python3.6-dev python3-pip python3.6-venv && \
     rm -rf /var/lib/apt/lists/* && \