When called by `_create_pseudo_member_()`, the dictionary iteration of `_value2member_map` in `_decompose()` in enum.py may lead to a "RuntimeError: dictionary changed size during iteration". For me, it happened in `re.compile`.

```
Traceback (most recent call last):
  ...
  File "/usr/local/Cellar/python3/3.6.0/Frameworks/Python.framework/Versions/3.6/lib/python3.6/re.py", line 233, in compile
    return _compile(pattern, flags)
  File "/usr/local/Cellar/python3/3.6.0/Frameworks/Python.framework/Versions/3.6/lib/python3.6/re.py", line 301, in _compile
    p = sre_compile.compile(pattern, flags)
  File "/usr/local/Cellar/python3/3.6.0/Frameworks/Python.framework/Versions/3.6/lib/python3.6/sre_compile.py", line 562, in compile
    p = sre_parse.parse(p, flags)
  File "/usr/local/Cellar/python3/3.6.0/Frameworks/Python.framework/Versions/3.6/lib/python3.6/sre_parse.py", line 866, in parse
    p.pattern.flags = fix_flags(str, p.pattern.flags)
  File "/usr/local/Cellar/python3/3.6.0/Frameworks/Python.framework/Versions/3.6/lib/python3.6/sre_parse.py", line 835, in fix_flags
    flags |= SRE_FLAG_UNICODE
  File "/usr/local/Cellar/python3/3.6.0/Frameworks/Python.framework/Versions/3.6/lib/python3.6/enum.py", line 794, in __or__
    result = self.__class__(self._value_ | self.__class__(other)._value_)
  File "/usr/local/Cellar/python3/3.6.0/Frameworks/Python.framework/Versions/3.6/lib/python3.6/enum.py", line 291, in __call__
    return cls.__new__(cls, value)
  File "/usr/local/Cellar/python3/3.6.0/Frameworks/Python.framework/Versions/3.6/lib/python3.6/enum.py", line 533, in __new__
    return cls._missing_(value)
  File "/usr/local/Cellar/python3/3.6.0/Frameworks/Python.framework/Versions/3.6/lib/python3.6/enum.py", line 760, in _missing_
    new_member = cls._create_pseudo_member_(value)
  File "/usr/local/Cellar/python3/3.6.0/Frameworks/Python.framework/Versions/3.6/lib/python3.6/enum.py", line 769, in _create_pseudo_member_
    _, extra_flags = _decompose(cls, value)
  File "/usr/local/Cellar/python3/3.6.0/Frameworks/Python.framework/Versions/3.6/lib/python3.6/enum.py", line 849, in _decompose
    for v, m in flag._value2member_map_.items()
  File "/usr/local/Cellar/python3/3.6.0/Frameworks/Python.framework/Versions/3.6/lib/python3.6/enum.py", line 848, in <listcomp>
    (m, v)
RuntimeError: dictionary changed size during iteration
```