ci: Use pyenv's python-build to install Python in lint task

.cirrus.yml

@@ -67,11 +67,14 @@ task:
   name: 'lint [bionic]'
   << : *BASE_TEMPLATE
   container:
-    image: ubuntu:bionic  # For python 3.6, oldest supported version according to doc/dependencies.md
+    image: ubuntu:bionic
     cpu: 1
     memory: 1G
   # For faster CI feedback, immediately schedule the linters
   << : *CREDITS_TEMPLATE
+  python_cache:
+    folder: "/tmp/python"
+    fingerprint_script: cat .python-version /etc/os-release
   lint_script:
     - ./ci/lint_run_all.sh
   env:

ci/lint/04_install.sh

@@ -7,7 +7,26 @@
 export LC_ALL=C
 
 ${CI_RETRY_EXE} apt-get update
-${CI_RETRY_EXE} apt-get install -y python3-pip curl git gawk jq
+${CI_RETRY_EXE} apt-get install -y curl git gawk jq xz-utils
+
+PYTHON_PATH=/tmp/python
+if [ ! -d "${PYTHON_PATH}/bin" ]; then
+  (
+    git clone https://github.com/pyenv/pyenv.git
+    cd pyenv/plugins/python-build || exit 1
+    ./install.sh
+  )
+  # For dependencies see https://github.com/pyenv/pyenv/wiki#suggested-build-environment
+  ${CI_RETRY_EXE} apt-get install -y build-essential libssl-dev zlib1g-dev \
+    libbz2-dev libreadline-dev libsqlite3-dev curl llvm \
+    libncursesw5-dev xz-utils tk-dev libxml2-dev libxmlsec1-dev libffi-dev liblzma-dev \
+    clang
+  env CC=clang python-build "$(cat "${BASE_ROOT_DIR}/.python-version")" "${PYTHON_PATH}"
+fi
+export PATH="${PYTHON_PATH}/bin:${PATH}"
+command -v python3
+python3 --version
+
 (
   # Temporary workaround for https://github.com/bitcoin/bitcoin/pull/26130#issuecomment-1260499544
   # Can be removed once the underlying image is bumped to something that includes git2.34 or later