Add code quality test for self assignments

There are test failures for PY3, related with string management.  An example:
```
============================================================================= test process starts ==============================================================================
executable:         /usr/bin/python3  (3.2.3-final-0) [CPython]
architecture:       64-bit
cache:              yes
ground types:       python 
random seed:        41586404
hash randomization: on (PYTHONHASHSEED=1079944393)

sympy/utilities/tests/test_code_quality.py[5] E....                                                                                                                       [FAIL]

________________________________________________________________________________________________________________________________________________________________________________
____________________________________________________________ sympy/utilities/tests/test_code_quality.py:test_files _____________________________________________________________
  File "/home/sk/src/sympy/sympy/sympy/utilities/tests/test_code_quality.py", line 235, in test_files
    check_directory_tree(SYMPY_PATH, test, exclude)
  File "/home/sk/src/sympy/sympy/sympy/utilities/tests/test_code_quality.py", line 110, in check_directory_tree
    check_files(glob(join(root, pattern)), file_check, exclusions)
  File "/home/sk/src/sympy/sympy/sympy/utilities/tests/test_code_quality.py", line 126, in check_files
    file_check(fname)
  File "/home/sk/src/sympy/sympy/sympy/utilities/tests/test_code_quality.py", line 148, in test
    result = find_self_assignments(fname)
  File "/home/sk/src/sympy/sympy/sympy/utilities/tests/test_code_quality.py", line 70, in find_self_assignments
    t = [n for n in ast.parse(source).body if isinstance(n, ast.ClassDef)]
  File "/usr/lib/python3.2/ast.py", line 36, in parse
    return compile(source, filename, mode, PyCF_ONLY_AST)
  File "<unknown>", line 1
    ﻿"""
      ^
SyntaxError: invalid character in identifier

=========================================================== tests finished: 4 passed, 1 exceptions, in 34.34 seconds ===========================================================
DO *NOT* COMMIT!
```

I don't know how to avoid this without importing unicode_literals.  But that's another big story.