With this commit, all ldap connections accept a new parameter,
``bytes_mode``.
When set to ``True``, this flag emulates the old Python 2 behavior,
where all fields are bytes - including those declared as UTF-8 by the RFC (DN,
RDN, attribute names).
If this flag is set to ``False``, the code works with text (unicode) for all
text fields (everything except attribute values).
If no value is set under Python 2, the code will raise a BytesWarning
and proceed with the flag set to ``True``, for backwards compatibility.
Under Python 3, the value can only be set to ``False``.
For safety and ease of upgrade, the code checks that all provided
arguments are of the expected type (unicode with ``bytes_mode=False``,
bytes with ``bytes_mode=True``).
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