Add .cirrus_requirements.txt

Closes #729

.cirrus.yml

@@ -4,8 +4,8 @@ container:
 code_check_task:
     pip_cache:
         folder: ~/.cache/pip
-        populate_script: pip install pylint requests yapf
-    pip_install_script: pip install pylint requests yapf
+        fingerprint_script: .cirrus_requirements
+        populate_script: pip install -r .cirrus_requirements.txt
     utils_script:
         - python3 -m yapf --style '.style.yapf' -e '*/third_party/*' -rpd utils
         - ./devutils/run_utils_pylint.py --hide-fixme

.cirrus_requirements.txt

@@ -0,0 +1,5 @@
+# Based on Python package versions in Debian buster
+astroid==2.1.0 # via pylint
+pylint==2.2.2
+requests==2.21.0
+yapf==0.25.0