I'm guessing this is a very seldom (never?) used code path.  I've included a patch to test and fix the problem.  The patch includes several related tests for pydoc.

$ py3 -c 'import pydoc; pydoc.synopsis("/opt/python3.4/lib/python3.4/lib-dynload/time.cpython-34m.so")'
Traceback (most recent call last):
  File "/opt/python3.4/lib/python3.4/tokenize.py", line 368, in find_cookie
    line_string = line.decode('utf-8')
UnicodeDecodeError: 'utf-8' codec can't decode byte 0xf4 in position 96: invalid continuation byte

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "/opt/python3.4/lib/python3.4/pydoc.py", line 229, in synopsis
    file = tokenize.open(filename)
  File "/opt/python3.4/lib/python3.4/tokenize.py", line 429, in open
    encoding, lines = detect_encoding(buffer.readline)
  File "/opt/python3.4/lib/python3.4/tokenize.py", line 409, in detect_encoding
    encoding = find_cookie(first)
  File "/opt/python3.4/lib/python3.4/tokenize.py", line 373, in find_cookie
    raise SyntaxError(msg)
SyntaxError: invalid or missing encoding declaration for '/opt/python3.4/lib/python3.4/lib-dynload/time.cpython-34m.so'