Simplify exception handling in is_ascii. (#9985)

We can get away with just catching UnicodeError here.

    ⋮
    +-- ValueError
    |    +-- UnicodeError
    |         +-- UnicodeDecodeError
    |         +-- UnicodeEncodeError
    |         +-- UnicodeTranslateError
    ⋮

https://docs.python.org/3/library/exceptions.html#exception-hierarchy

Signed-off-by: Dan Callahan <danc@element.io>

changelog.d/9985.misc

@@ -0,0 +1 @@
+Simplify a few helper functions.

synapse/util/stringutils.py

@@ -55,9 +55,7 @@ def random_string_with_symbols(length: int) -> str:
 def is_ascii(s: bytes) -> bool:
     try:
         s.decode("ascii").encode("ascii")
-    except UnicodeDecodeError:
-        return False
-    except UnicodeEncodeError:
+    except UnicodeError:
         return False
     return True