tell circle to use python 3.7

...since pycrypto doesn't support 3.8 :trollface:

.circleci/config.yml

@@ -7,7 +7,7 @@ version: 2
 jobs:
   build:
     docker:
-      # https://circleci.com/docs/2.0/google-container-engine/#selecting-a-base-image
+      # https://circleci.com/docs/2.0/google-auth/#installing-the-google-cloud-sdk
       - image: google/cloud-sdk
 
     environment:
@@ -21,16 +21,11 @@ jobs:
           key: venv-{{ .Branch }}-{{ checksum "requirements.txt" }}
 
       - run:
-          name: Base dependencies
+          name: Dependencies
           command: |
             apt-get update
-            apt-get install -y python3
-
-      - run:
-          name: Python 3 dependencies
-          command: |
-            apt-get install -y python3-venv python3-dev
-            python3 -m venv local3
+            apt-get install -y python3.7 python3.7-venv python3.7-dev
+            python3.7 -m venv local3
             . local3/bin/activate
             pip install -U -r requirements.txt
             pip install mox3