Log Message:
py-ZopeSchema: updated to 4.9.3

4.9.3:
- Fixed a ReST error in getDoc() results when having "subfields"
  with titles.

4.9.2:
- Make sure that the title for IObject.validate_invariants is a unicode
  string.

4.9.1:
- Fix SimpleTerm token for non-ASCII bytes values.

4.9.0:
- Make NativeString and NativeStringLine distinct types that
  implement the newly-distinct interfaces INativeString and
  INativeStringLine. Previously these were just aliases for either
  Text (on Python 3) or Bytes (on Python 2).
- Fix Field.getDoc() when value_type or key_type is
  present. Previously it could produce ReST that generated Sphinx
  warnings.
- Make DottedName accept leading underscores for each segment.
- Add PythonIdentifier, which accepts one segment of a dotted
  name, e.g., a python variable or class.

4.8.0:
- Add the interface IFromBytes, which is implemented by the
  numeric and bytes fields, as well as URI, DottedName, and
  Id.
- Fix passing None as the description to a field constructor.