Only build python 3.7 and say so in the Pipfile too

.github/workflows/docker-publish.yml

@@ -10,7 +10,7 @@ jobs:
     strategy:
       matrix:
         os: [ubuntu-20.04]
-        python-version: [3.7, 3.8, 3.9]
+        python-version: [3.7]
     steps:
     - uses: actions/checkout@v2
     - name: Set up Python ${{ matrix.python-version }}

Pipfile

@@ -9,7 +9,7 @@ verify_ssl = true
 name = "piwheels"
 
 [requires]
-python_version = "3.6"
+python_version = "3.7"
 
 [packages]
 dateparser = "~=0.7.6"