text: Title: Django improperly configured error if secret key begins with a dollar sign character
  Body: I've come across this error which only seems to occur when using the django-environ package to read a SECRET_KEY from a .env file.
  
  My .env file (with a generated secret key using the get_random_secret_key module from django)
  
  `SECRET_KEY=$q*eo+c&2!)u9^tpd6f=0szxt6+th!j^#z9$1mh!kyen*36($t)`
  
  My settings.py file
  
  ```
  import environ
  env = environ.Env()
  environ.Env.read_env()
  
  SECRET_KEY = env('SECRET_KEY')
  ```
  
  Running this code will generate the following error
  
  `django.core.exceptions.ImproperlyConfigured: Set the q*eo+c&2!)u9^tpd6f=0szxt6+th!j^#z9$1mh!kyen*36($t) environment variable`
  
  Even if I surround the secret key value with single or double quotes the error will still occur.
  
  This error does not occur if I use a different python environment file package such as python-dotenv to read the same .env file.
  
  The django-environ package seems to think the SECRET_KEY value is an environment variable itself. 
  I know this is a very niche situation as this secret key that I generated just happened to have a leading '$' character but it may catch people out if they're generating their secret key with a script. The error occurs for any secret key value that starts with a '$'.
  Full stack trace:
  
  ```
  Traceback (most recent call last):
    File "/opt/anaconda3/envs/django_py/lib/python3.9/site-packages/environ/environ.py", line 273, in get_value
      value = self.ENVIRON[var]
    File "/opt/anaconda3/envs/django_py/lib/python3.9/os.py", line 679, in __getitem__
      raise KeyError(key) from None
  KeyError: 'q*eo+c&2!)u9^tpd6f=0szxt6+th!j^#z9$1mh!kyen*36($t)'
  
  During handling of the above exception, another exception occurred:
  
  Traceback (most recent call last):
    File "/Users/danieloram/Desktop/test_env/environ_secret_key.py", line 10, in <module>
      SECRET_KEY = env('SECRET_KEY')
    File "/opt/anaconda3/envs/django_py/lib/python3.9/site-packages/environ/environ.py", line 123, in __call__
      return self.get_value(var, cast=cast, default=default, parse_default=parse_default)
    File "/opt/anaconda3/envs/django_py/lib/python3.9/site-packages/environ/environ.py", line 284, in get_value
      value = self.get_value(value, cast=cast, default=default)
    File "/opt/anaconda3/envs/django_py/lib/python3.9/site-packages/environ/environ.py", line 277, in get_value
      raise ImproperlyConfigured(error_msg)
  django.core.exceptions.ImproperlyConfigured: Set the q*eo+c&2!)u9^tpd6f=0szxt6+th!j^#z9$1mh!kyen*36($t) environment variable
  ```
  
  django version: 3.0.3
  
  django-environ version: 0.4.5