build(python): remove python 3.7 from kokoro Dockerfile

.github/.OwlBot.lock.yaml

@@ -1,3 +1,3 @@
 docker:
   image: gcr.io/repo-automation-bots/owlbot-python:latest
-  digest: sha256:e1793a23ae0ee9aafb2e3a53b564a351f74790dbe3c2d75f8fc3b8c43e5c036c
+  digest: sha256:5ff7446edeaede81c3ed58b23a4e76a5403fba1350ce28478045657303b6479d

.kokoro/docker/docs/Dockerfile

@@ -40,6 +40,7 @@ RUN apt-get update \
     libssl-dev \
     libsqlite3-dev \
     portaudio19-dev \
+    python3-distutils \
     redis-server \
     software-properties-common \
     ssh \
@@ -59,40 +60,8 @@ RUN apt-get update \
   && rm -rf /var/lib/apt/lists/* \
   && rm -f /var/cache/apt/archives/*.deb
 
-
-COPY fetch_gpg_keys.sh /tmp
-# Install the desired versions of Python.
-RUN set -ex \
-    && export GNUPGHOME="$(mktemp -d)" \
-    && echo "disable-ipv6" >> "${GNUPGHOME}/dirmngr.conf" \
-    && /tmp/fetch_gpg_keys.sh \
-    && for PYTHON_VERSION in 3.7.8 3.8.5; do \
-        wget --no-check-certificate -O python-${PYTHON_VERSION}.tar.xz "https://www.python.org/ftp/python/${PYTHON_VERSION%%[a-z]*}/Python-$PYTHON_VERSION.tar.xz" \
-        && wget --no-check-certificate -O python-${PYTHON_VERSION}.tar.xz.asc "https://www.python.org/ftp/python/${PYTHON_VERSION%%[a-z]*}/Python-$PYTHON_VERSION.tar.xz.asc" \
-        && gpg --batch --verify python-${PYTHON_VERSION}.tar.xz.asc python-${PYTHON_VERSION}.tar.xz \
-        && rm -r python-${PYTHON_VERSION}.tar.xz.asc \
-        && mkdir -p /usr/src/python-${PYTHON_VERSION} \
-        && tar -xJC /usr/src/python-${PYTHON_VERSION} --strip-components=1 -f python-${PYTHON_VERSION}.tar.xz \
-        && rm python-${PYTHON_VERSION}.tar.xz \
-        && cd /usr/src/python-${PYTHON_VERSION} \
-        && ./configure \
-            --enable-shared \
-            # This works only on Python 2.7 and throws a warning on every other
-            # version, but seems otherwise harmless.
-            --enable-unicode=ucs4 \
-            --with-system-ffi \
-            --without-ensurepip \
-        && make -j$(nproc) \
-        && make install \
-        && ldconfig \
-  ; done \
-  && rm -rf "${GNUPGHOME}" \
-  && rm -rf /usr/src/python* \
-  && rm -rf ~/.cache/
-
 RUN wget -O /tmp/get-pip.py 'https://bootstrap.pypa.io/get-pip.py' \
-  && python3.7 /tmp/get-pip.py \
   && python3.8 /tmp/get-pip.py \
   && rm /tmp/get-pip.py
 
-CMD ["python3.7"]
+CMD ["python3.8"]