Add support for building Python 3 Documentation Info manual

[kaushalmodi]

To build Python 3.6.1 Info manual, do:

make install3 PYTHON_VERSION=3.6.1 INFO_DIR=/path/to/my/info/dir

Also add python3.info and python3.texi for Python 3.6.1.

I am not an expert in writing Makefiles, so I believe my implementation is a bit crude (duplication of existing Makefile targets with 3 suffix), but it works :)

[kaushalmodi]

Also note that I get these warnings in the makeinfo step, but I don't know how to resolve them. The generated Python 3 Info manual is still very much usable.

makeinfo --no-split python3.texi
python3.texi:24440: warning: @subsection missing argument
python3.texi:64097: warning: could not find @image file `win_installer.txt' nor alternate text
python3.texi:115007: warning: could not find @image file `pathlib-inheritance.txt' nor alternate text
python3.texi:172863: warning: could not find @image file `tulip_coro.txt' nor alternate text
python3.texi:217555: warning: could not find @image file `turtle-star.txt' nor alternate text
python3.texi:299273: warning: could not find @image file `logging_flow.txt' nor alternate text

[kaushalmodi]

@Wilfred Can you please review this?

[snowman]

Please don't commit python3.info and python3.texi file?

[kaushalmodi]

I'll just close this as there hasn't been any response from the repo owner.

[alejandrogallo]

for 3.9.9 is working https://github.com/alejandrogallo/python-info