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This is a regression caused by the new HTTPStatus enum from Issue 21793. RFC 6585 uses the plural "Fields", so maybe the new enum symbol needs renaming.
$ python3.4
Python 3.4.2 (default, Oct 8 2014, 13:44:52) 
[GCC 4.9.1 20140903 (prerelease)] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import http.client
>>> http.client.REQUEST_HEADER_FIELDS_TOO_LARGE
431
>>> http.client.REQUEST_HEADER_FIELD_TOO_LARGE
Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
AttributeError: 'module' object has no attribute 'REQUEST_HEADER_FIELD_TOO_LARGE'
>>> 
$ ./python
Python 3.5.0a0 (qbase qtip simple-http-testing.patch tip:f3fadbfb10ba, Feb 11 2015, 07:18:07) 
[GCC 4.9.1] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import http.client
>>> http.client.REQUEST_HEADER_FIELDS_TOO_LARGE
Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
AttributeError: module 'http.client' has no attribute 'REQUEST_HEADER_FIELDS_TOO_LARGE'
>>> http.client.REQUEST_HEADER_FIELD_TOO_LARGE
<HTTPStatus.REQUEST_HEADER_FIELD_TOO_LARGE: 431>