build(python): remove python 3.7 from kokoro Dockerfile (#1155)

A lot of python repositories have a failing `Kokoro docs-presubmit` check with error `gpg: keyserver receive failed: No name`. For example, googleapis/python-asset#227

The issue is that the `fetch_gpg_keys.sh` script is failing in the Docker file [here](https://github.com/googleapis/synthtool/blob/master/synthtool/gcp/templates/python_library/.kokoro/docker/docs/Dockerfile#L68) because the key [here](https://github.com/googleapis/synthtool/blob/master/synthtool/gcp/templates/python_library/.kokoro/docker/docs/fetch_gpg_keys.sh#L39) is no longer available on the keyserver.

This PR removes `fetch_gpg_keys.sh` as, in most repositories, we don't need to install additional python versions. We can use the default python version in the ubuntu 20.04 base image which is python 3.8.10.

synthtool/gcp/templates/python_library/.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"]

tests/test_python_library.py

@@ -108,7 +108,7 @@ def test_python_library():
         templated_files = common.py_library()
 
         assert os.path.exists(templated_files / ".kokoro/docs/docs-presubmit.cfg")
-        assert os.path.exists(templated_files / ".kokoro/docker/docs/fetch_gpg_keys.sh")
+        assert os.path.exists(templated_files / ".kokoro/docker/docs/Dockerfile")
 
 
 def test_split_system_tests():