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_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):