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/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