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

tests/functional/test_migration_linter.py

@@ -8,7 +8,7 @@ from django_migration_linter import MigrationLinter
 from tests import fixtures
 
 
-class BaseBackwardCompatibilityDetection(object):
+class BaseBackwardCompatibilityDetection:
     def setUp(self, *args, **kwargs):
         self.database = next(iter(self.databases))
         self.test_project_path = os.path.dirname(settings.BASE_DIR)
@@ -24,7 +24,7 @@ class BaseBackwardCompatibilityDetection(object):
             "app_unique_together",
             database=self.database,
         )
-        return super(BaseBackwardCompatibilityDetection, self).setUp(*args, **kwargs)
+        return super().setUp(*args, **kwargs)
 
     def _test_linter_finds_errors(self, app=None, commit_id=None):
         linter = self._launch_linter(app, commit_id)

tests/test_project/app_add_not_null_column_followed_by_default/migrations/0001_initial.py

@@ -1,4 +1,3 @@
-# -*- coding: utf-8 -*-
 # Generated by Django 1.11.18 on 2019-04-14 14:54
 
 

tests/test_project/app_add_not_null_column_followed_by_default/migrations/0002_a_not_null_field.py

@@ -1,4 +1,3 @@
-# -*- coding: utf-8 -*-
 # Generated by Django 1.11.18 on 2019-04-14 14:55
 
 

tests/test_project/app_add_not_null_column_followed_by_default/models.py

@@ -1,6 +1,3 @@
-# -*- coding: utf-8 -*-
-
-
 from django.db import models
 
 

tests/test_project/app_alter_column/migrations/0001_initial.py

@@ -1,4 +1,3 @@
-# -*- coding: utf-8 -*-
 # Generated by Django 1.11.18 on 2019-04-14 14:56
 
 

tests/test_project/app_alter_column/migrations/0002_auto_20190414_1456.py

@@ -1,4 +1,3 @@
-# -*- coding: utf-8 -*-
 # Generated by Django 1.11.18 on 2019-04-14 14:56
 
 

tests/test_project/app_alter_column/models.py

@@ -1,6 +1,3 @@
-# -*- coding: utf-8 -*-
-
-
 from django.db import models
 
 

tests/test_project/app_create_table_with_not_null_column/migrations/0001_initial.py

@@ -1,4 +1,3 @@
-# -*- coding: utf-8 -*-
 # Generated by Django 1.11.18 on 2019-04-14 14:59
 
 

tests/test_project/app_create_table_with_not_null_column/models.py

@@ -1,6 +1,3 @@
-# -*- coding: utf-8 -*-
-
-
 from django.db import models
 
 

tests/test_project/app_drop_column/migrations/0001_initial.py

@@ -1,4 +1,3 @@
-# -*- coding: utf-8 -*-
 # Generated by Django 1.11.18 on 2019-04-14 14:59
 
 

tests/test_project/app_drop_column/migrations/0002_remove_a_field_b.py

@@ -1,4 +1,3 @@
-# -*- coding: utf-8 -*-
 # Generated by Django 1.11.18 on 2019-04-14 14:59
 
 

tests/test_project/app_drop_column/models.py

@@ -1,6 +1,3 @@
-# -*- coding: utf-8 -*-
-
-
 from django.db import models
 
 

tests/test_project/app_drop_table/migrations/0001_initial.py

@@ -1,4 +1,3 @@
-# -*- coding: utf-8 -*-
 # Generated by Django 1.11.18 on 2019-04-14 14:59
 
 

tests/test_project/app_drop_table/models.py

@@ -1,6 +1,3 @@
-# -*- coding: utf-8 -*-
-
-
 from django.db import models
 
 # class A(models.Model):

tests/test_project/app_rename_column/migrations/0001_initial.py

@@ -1,4 +1,3 @@
-# -*- coding: utf-8 -*-
 # Generated by Django 1.11.18 on 2019-04-14 15:02
 
 

tests/test_project/app_rename_column/migrations/0002_auto_20190414_1502.py

@@ -1,4 +1,3 @@
-# -*- coding: utf-8 -*-
 # Generated by Django 1.11.18 on 2019-04-14 15:02
 
 

tests/test_project/app_rename_column/models.py

@@ -1,6 +1,3 @@
-# -*- coding: utf-8 -*-
-
-
 from django.db import models
 
 

tests/test_project/app_rename_table/migrations/0001_initial.py

@@ -1,4 +1,3 @@
-# -*- coding: utf-8 -*-
 # Generated by Django 1.11.18 on 2019-04-14 15:00
 
 

tests/test_project/app_rename_table/migrations/0002_auto_20190414_1500.py

@@ -1,4 +1,3 @@
-# -*- coding: utf-8 -*-
 # Generated by Django 1.11.18 on 2019-04-14 15:00
 
 

tests/test_project/app_rename_table/models.py

@@ -1,6 +1,3 @@
-# -*- coding: utf-8 -*-
-
-
 from django.db import models
 
 

tests/unit/test_sql_analyser.py

@@ -19,10 +19,10 @@ class SqlAnalyserTestCase(unittest.TestCase):
 
     def assertValidSql(self, sql, allow_warnings=False):
         errors, _, warnings = self.analyse_sql(sql)
-        self.assertEqual(0, len(errors), "Found errors in sql: {}".format(errors))
+        self.assertEqual(0, len(errors), f"Found errors in sql: {errors}")
         if not allow_warnings:
             self.assertEqual(
-                0, len(warnings), "Found warnings in sql: {}".format(errors)
+                0, len(warnings), f"Found warnings in sql: {errors}"
             )
 
     def assertBackwardIncompatibleSql(self, sql, code=None):