Fix "publish heroku" + upgrade to use Python 3.8.0

Closes #633. Closes #632.

datasette/publish/heroku.py

@@ -72,7 +72,10 @@ def publish_subcommand(publish):
             "about_url": about_url,
         }
 
-        environment_variables = {}
+        environment_variables = {
+            # Avoid uvicorn error: https://github.com/simonw/datasette/issues/633
+            "WEB_CONCURRENCY": "1"
+        }
         if plugin_secret:
             extra_metadata["plugins"] = {}
             for plugin_name, plugin_setting, setting_value in plugin_secret:
@@ -164,7 +167,7 @@ def temporary_heroku_directory(
         if metadata_content:
             open("metadata.json", "w").write(json.dumps(metadata_content, indent=2))
 
-        open("runtime.txt", "w").write("python-3.6.8")
+        open("runtime.txt", "w").write("python-3.8.0")
 
         if branch:
             install = [

tests/test_publish_heroku.py

@@ -57,8 +57,13 @@ def test_publish_heroku(mock_call, mock_check_output, mock_which):
         open("test.db", "w").write("data")
         result = runner.invoke(cli.cli, ["publish", "heroku", "test.db"])
         assert 0 == result.exit_code, result.output
-        mock_call.assert_called_once_with(
-            ["heroku", "builds:create", "-a", "f", "--include-vcs-ignore"]
+        mock_call.assert_has_calls(
+            [
+                mock.call(["heroku", "config:set", "-a", "f", "WEB_CONCURRENCY=1",]),
+                mock.call(
+                    ["heroku", "builds:create", "-a", "f", "--include-vcs-ignore"]
+                ),
+            ]
         )