Remote dependency on distutils (#9125)

`distutils` is pretty much deprecated these days, and replaced with
`setuptools`. It's also annoying because it's you can't `pip install` it, and
it's hard to figure out which debian package we should depend on to make sure
it's there.

Since we only use it for a tiny function anyway, let's just vendor said
function into our codebase.

synapse/util/stringutils.py

@@ -75,3 +75,22 @@ def shortstr(iterable: Iterable, maxitems: int = 5) -> str:
     if len(items) <= maxitems:
         return str(items)
     return "[" + ", ".join(repr(r) for r in items[:maxitems]) + ", ...]"
+
+
+def strtobool(val: str) -> bool:
+    """Convert a string representation of truth to True or False
+
+    True values are 'y', 'yes', 't', 'true', 'on', and '1'; false values
+    are 'n', 'no', 'f', 'false', 'off', and '0'.  Raises ValueError if
+    'val' is anything else.
+
+    This is lifted from distutils.util.strtobool, with the exception that it actually
+    returns a bool, rather than an int.
+    """
+    val = val.lower()
+    if val in ("y", "yes", "t", "true", "on", "1"):
+        return True
+    elif val in ("n", "no", "f", "false", "off", "0"):
+        return False
+    else:
+        raise ValueError("invalid truth value %r" % (val,))