text: Title: url "schema" should be "scheme"
  Body: https://github.com/psf/requests/blob/590350f8d094c216051510ed1dd18fe871b53b72/requests/models.py#L388-L392
  
  I don't believe the first part of a URL is ever called a "schema." Exceptions and error messages referring to an incorrect schema are confusing, especially in contexts where actual schema errors are possible. If it isn't possible to change the `MissingSchema` exception (it looks like it is slated to be fixed in 3.x) please consider at least changing the error message.
  
  References:
  https://www.w3.org/Addressing/URL/url-spec.txt
  https://github.com/psf/requests/issues/4495