docs: kdoc: handle the obsolescensce of docutils.ErrorString()

The ErrorString() and SafeString() docutils functions were helpers meant to
ease the handling of encodings during the Python 3 transition.  There is no
real need for them after Python 3.6, and docutils 0.22 removes them,
breaking the docs build

Handle this by just injecting our own one-liner version of ErrorString(),
and removing the sole SafeString() call entirely.

Reported-by: Zhixu Liu <zhixu.liu@gmail.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Message-ID: <87ldmnv2pi.fsf@trenco.lwn.net>

Documentation/sphinx/kernel_include.py

@@ -89,7 +89,6 @@ import sys
 
 from docutils import io, nodes, statemachine
 from docutils.statemachine import ViewList
-from docutils.utils.error_reporting import SafeString, ErrorString
 from docutils.parsers.rst import Directive, directives
 from docutils.parsers.rst.directives.body import CodeBlock, NumberLines
 
@@ -106,6 +105,9 @@ logger = logging.getLogger(__name__)
 RE_DOMAIN_REF = re.compile(r'\\ :(ref|c:type|c:func):`([^<`]+)(?:<([^>]+)>)?`\\')
 RE_SIMPLE_REF = re.compile(r'`([^`]+)`')
 
+def ErrorString(exc):  # Shamelessly stolen from docutils
+    return f'{exc.__class__.__name}: {exc}'
+
 
 # ==============================================================================
 class KernelInclude(Directive):
@@ -156,7 +158,7 @@ class KernelInclude(Directive):
             except UnicodeEncodeError:
                 raise self.severe('Problems with directive path:\n'
                                 'Cannot encode input file path "%s" '
-                                '(wrong locale?).' % SafeString(path))
+                                '(wrong locale?).' % path)
             except IOError as error:
                 raise self.severe('Problems with directive path:\n%s.' % ErrorString(error))