Fix default Python version declaration in pre-commit config (#584)

* Fix default Python version declaration in pre-commit config

The new way of of declaring default Python version was introduced in
pre-commit 1.14.0 and warning about unknown config keys was introduced in
pre-commit 1.17.0.

.pre-commit-config.yaml

@@ -35,4 +35,5 @@ repos:
     hooks:
     -   id: check-hooks-apply
     -   id: check-useless-excludes
-python_version: python3.6
+default_language_version:
+    python: python3.7

.taskcluster.yml

@@ -35,7 +35,7 @@ tasks:
         workerType: github-worker
         payload:
           maxRunTime: 3600
-          image: python
+          image: python:3.7
           command:
             - "/bin/bash"
             - "-lcx"

setup.py

@@ -50,7 +50,7 @@ setup(
         ]
     },
     classifiers=[
-        "Programming Language :: Python :: 3.6",
+        "Programming Language :: Python :: 3.7",
         "Programming Language :: Python :: 3 :: Only",
         "License :: OSI Approved :: Mozilla Public License 2.0 (MPL 2.0)",
     ],