ImportError: No module named 'MySQLdb'の対処法

mysqlを使ってdjango起動時にImportError: No module named 'MySQLdb'が発生したので、その時の対処法。

```
ImportError: No module named 'MySQLdb'

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "/usr/local/lib/python3.5/dist-packages/django/utils/autoreload.py", line 225, in wrapper
    fn(*args, **kwargs)
  File "/usr/local/lib/python3.5/dist-packages/django/core/management/commands/runserver.py", line 109, in inner_run
    autoreload.raise_last_exception()
  File "/usr/local/lib/python3.5/dist-packages/django/utils/autoreload.py", line 248, in raise_last_exception
    raise _exception[1]
  File "/usr/local/lib/python3.5/dist-packages/django/core/management/__init__.py", line 337, in execute
    autoreload.check_errors(django.setup)()
  File "/usr/local/lib/python3.5/dist-packages/django/utils/autoreload.py", line 225, in wrapper
    fn(*args, **kwargs)
  File "/usr/local/lib/python3.5/dist-packages/django/__init__.py", line 24, in setup
    apps.populate(settings.INSTALLED_APPS)
  File "/usr/local/lib/python3.5/dist-packages/django/apps/registry.py", line 112, in populate
    app_config.import_models()
  File "/usr/local/lib/python3.5/dist-packages/django/apps/config.py", line 198, in import_models
    self.models_module = import_module(models_module_name)
  File "/usr/lib/python3.5/importlib/__init__.py", line 126, in import_module
    return _bootstrap._gcd_import(name[level:], package, level)
  File "<frozen importlib._bootstrap>", line 986, in _gcd_import
  File "<frozen importlib._bootstrap>", line 969, in _find_and_load
  File "<frozen importlib._bootstrap>", line 958, in _find_and_load_unlocked
  File "<frozen importlib._bootstrap>", line 673, in _load_unlocked
  File "<frozen importlib._bootstrap_external>", line 665, in exec_module
  File "<frozen importlib._bootstrap>", line 222, in _call_with_frames_removed
  File "/usr/local/lib/python3.5/dist-packages/django/contrib/auth/models.py", line 2, in <module>
    from django.contrib.auth.base_user import AbstractBaseUser, BaseUserManager
  File "/usr/local/lib/python3.5/dist-packages/django/contrib/auth/base_user.py", line 47, in <module>
    class AbstractBaseUser(models.Model):
  File "/usr/local/lib/python3.5/dist-packages/django/db/models/base.py", line 101, in __new__
    new_class.add_to_class('_meta', Options(meta, app_label))
  File "/usr/local/lib/python3.5/dist-packages/django/db/models/base.py", line 305, in add_to_class
    value.contribute_to_class(cls, name)
  File "/usr/local/lib/python3.5/dist-packages/django/db/models/options.py", line 203, in contribute_to_class
    self.db_table = truncate_name(self.db_table, connection.ops.max_name_length())
  File "/usr/local/lib/python3.5/dist-packages/django/db/__init__.py", line 33, in __getattr__
    return getattr(connections[DEFAULT_DB_ALIAS], item)
  File "/usr/local/lib/python3.5/dist-packages/django/db/utils.py", line 202, in __getitem__
    backend = load_backend(db['ENGINE'])
  File "/usr/local/lib/python3.5/dist-packages/django/db/utils.py", line 110, in load_backend
    return import_module('%s.base' % backend_name)
  File "/usr/lib/python3.5/importlib/__init__.py", line 126, in import_module
    return _bootstrap._gcd_import(name[level:], package, level)
  File "/usr/local/lib/python3.5/dist-packages/django/db/backends/mysql/base.py", line 20, in <module>
    ) from err
django.core.exceptions.ImproperlyConfigured: Error loading MySQLdb module.
Did you install mysqlclient?
```

いろいろ試したけど、これで解決した。

```
$ sudo apt-get install python3-mysqldb
```