Black runs into internal error when docstring ends with EM QUAD

[Jackenmen]

Describe the bug

To Reproduce

1. Copy this code (it needs to be copied as it contains \u2001 character at the end of first docstring line):

def foo():
    """A special character: ` 
`
    """

2. Run Black on it with the default configuration.
3. See error:

INTERNAL ERROR: Black produced code that is not equivalent to the source.  Please report a bug on https://github.com/psf/black/issues.  This diff might be helpful: /tmp/blk_o610r37n.log

--- src
+++ dst
@@ -16,11 +16,11 @@
       body=
         Expr(
           value=
             Constant(
               value=
-                'A special character: `\u2001\n`',  # str
+                'A special character: `\n`',  # str
             )  # /Constant
         )  # /Expr
       decorator_list=
       name=
         'foo',  # str

Expected behavior
No error

Environment:

• Version: both stable and master
• OS and Python version: Windows/Python 3.8 (as well as whatever is running on Black Playground)

Does this bug also happen on master? Yes

Additional context
None

[JelleZijlstra]

This is https://www.compart.com/en/unicode/U+2001, a whitespace character, so presumably we're stripping it as trailing whitespace in the docstring processing code, but not accepting the change in the safety check. Shouldn't be hard to fix.