TWO POINTS:
(1) I am not very concerned about chars like \x9d which are not valid in
the declared encoding; I am more concerned with chars like \x93 and \x94
which *ARE* valid in the declared encoding. Please ensure that these
cases are included in tests.
(2) Please check your test data and test results. I get different
results. I have created a file x9d.py by making the minimal changes to
x94.py. For me, this blows up on bytecompiling with *both* 3.0
(UnicodeDecodeError, as expected) and 2.x (Syntax Error unknown encoding
cp1252, wrong message) -- see below.

byte-compiling C:\python30\Lib\site-packages\x9d.py to x9d.pyc
Traceback (most recent call last):
  File "setup.py", line 5, in <module>
    py_modules = ["foo3", "bar3", "x93", "x94", "x9d", "xa0b7"]
  File "C:\python30\lib\distutils\core.py", line 149, in setup
    dist.run_commands()
  File "C:\python30\lib\distutils\dist.py", line 942, in run_commands
    self.run_command(cmd)
  File "C:\python30\lib\distutils\dist.py", line 962, in run_command
    cmd_obj.run()
  File "C:\python30\lib\distutils\command\install.py", line 571, in run
    self.run_command(cmd_name)
  File "C:\python30\lib\distutils\cmd.py", line 317, in run_command
    self.distribution.run_command(command)
  File "C:\python30\lib\distutils\dist.py", line 962, in run_command
    cmd_obj.run()
  File "C:\python30\lib\distutils\command\install_lib.py", line 91, in run
    self.byte_compile(outfiles)
  File "C:\python30\lib\distutils\command\install_lib.py", line 125, in
byte_compile
    dry_run=self.dry_run)
  File "C:\python30\lib\distutils\util.py", line 520, in byte_compile
    compile(file, cfile, dfile)
  File "C:\python30\lib\py_compile.py", line 137, in compile
    codestring = f.read()
  File "C:\python30\lib\io.py", line 1724, in read
    decoder.decode(self.buffer.read(), final=True))
  File "C:\python30\lib\io.py", line 1295, in decode
    output = self.decoder.decode(input, final=final)
  File "C:\python30\lib\encodings\cp1252.py", line 23, in decode
    return codecs.charmap_decode(input,self.errors,decoding_table)[0]
UnicodeDecodeError: 'charmap' codec can't decode byte 0x9d in position
64: character maps to <undefined>

byte-compiling C:\python26\Lib\site-packages\x9d.py to x9d.pyc
SyntaxError: ('unknown encoding: cp1252',
('C:\\python26\\Lib\\site-packages\\x9d.py', 0, 0, None))

byte-compiling c:\python25\Lib\site-packages\x9d.py to x9d.pyc
  File "c:\python25\Lib\site-packages\x9d.py", line 0
SyntaxError: ('unknown encoding: cp1252',
('c:\\python25\\Lib\\site-packages\\x9d.py', 0, 0, None))