chore: Upgrade Python syntax with pyupgrade

Run with the following pre-commit configuration:

```
repos:
  - repo: https://github.com/asottile/pyupgrade
    rev: v3.1.0
    hooks:
      - id: pyupgrade
        args: [ "--py3-plus", "--py36-plus", "--py37-plus" ]
```

The rationale for every change can be found at https://github.com/asottile/pyupgrade

setup.py

@@ -10,7 +10,7 @@ PROJECT_DIR = path.abspath(path.dirname(__file__))
 def get_version():
     constants = path.join(PROJECT_DIR, "django_migration_linter", "constants.py")
     _version_re = re.compile(r"__version__\s+=\s+(?P<version>.*)")
-    with open(constants, "r") as f:
+    with open(constants) as f:
         match = _version_re.search(f.read())
         version = match.group("version") if match is not None else '"unknown"'
     return str(ast.literal_eval(version))