test1.py shows that a str literal before first import from __future__ is a docstring.
test2.py shows that a str literal after first import from __future__ is not a docstring.
test2.py shows that if docstring is absent, then a single str literal between imports from __future__ does not cause SyntaxError, while it should.
test3.py shows that if docstring is present, then a str literal between imports from __future__ causes SyntaxError.
test4.py shows that if docstring is absent, then >=2 str literals between imports from __future__ cause SyntaxError.

$ cat test1.py
"some text"
from __future__ import absolute_import
print(__doc__)
$ cat test2.py
from __future__ import absolute_import
"some text"
from __future__ import print_function
print(__doc__)
$ cat test3.py
"some text 1"
from __future__ import absolute_import
"some text 2"
from __future__ import print_function
$ cat test4.py
from __future__ import absolute_import
"some text 1"
"some text 2"
from __future__ import print_function
$ python3.4 test1.py
some text
$ python3.4 test2.py
None
$ python3.4 test3.py
  File "test3.py", line 4
    from __future__ import print_function
    ^
SyntaxError: from __future__ imports must occur at the beginning of the file
$ python3.4 test4.py
  File "test4.py", line 4
    from __future__ import print_function
    ^
SyntaxError: from __future__ imports must occur at the beginning of the file
$

Jython and PyPy properly raise SyntaxError for test2.py.

46cadd3955d0

New changeset 46cadd3955d0 by Benjamin Peterson in branch 'default':
reject non-docs strings between future imports (closes #17434)
http://hg.python.org/cpython/rev/46cadd3955d0