Describe the bug

Removal of new line in docstrings can violate the line length constraint if the last line is within two characters of the max line length value.

To Reproduce Steps to reproduce the behavior:

Run black on the following code with the default line-length constraint of 88 characters.

def f():
    """89 123456789 123456789 123456789 123456789 123456789 123456789 123456789 12345678
    """
    pass

Expected behavior

No reformatting is performed.

Actual behavior

The code is reformatted as shown below, which violates the max line length constraint.

def f():
    """89 123456789 123456789 123456789 123456789 123456789 123456789 123456789 12345678"""
    pass

In a CI setup with Flake8 running after black, Flake8 throws the following error:

test.py:2:89: E501 line too long (91 > 88 characters)

Environment (please complete the following information):

• All versions of black from 20.8b0 up to 21.5b1 as well as main (d3670d9).
• Reproduced on Windows 10 & Ubuntu 18.04/20.04, Python 3.7 & 3.8

Does this bug also happen on main?

Yes

Additional context

black 19.10b0 does not have this behavior.
This was reported in #1635 (comment).