MAINT: Replace optparse with argparse for 'doc' and 'tools' scripts

Signed-off-by: Changqing Li <changqing.li@windriver.com>

* Cleanup unused imports (F401) of mostly standard Python modules,
  or some internal but unlikely referenced modules
* Where internal imports are potentially used, mark with noqa
* Avoid redefinition of imports (F811)

As numpy is Python 3 only, these import statements are now unnecessary
and don't alter runtime behavior.

Now is as good a time as any with open PR's at a low.

The recent Python 3 fix adding encoding="utf-8" to the open function
fails on Python 2 because the encoding keyword is not defined. The
solution is to use io.open, which is available for Python 2.6 and 2.7
and is an alias for open when Python >= 3.0. It is reputed to be slow
when running in 2.6, but that should not be a problem.

The default (ascii) encoding breaks:

python3.3 postprocess.py html build/html/*.html
Traceback (most recent call last):
  File "postprocess.py", line 59, in <module>
    main()
  File "postprocess.py", line 27, in main
    lines = process_html(fn, f.readlines())
  File "/usr/lib64/python3.3/encodings/ascii.py", line 26, in

    return codecs.ascii_decode(input, self.errors)[0]
UnicodeDecodeError: 'ascii' codec can't decode byte 0xc2 in
2309: ordinal not in range(128)

Add `print_function` to all `from __future__ import ...` statements
and use the python3 print function syntax everywhere.

Closes #3078.