It looks like this was caused by an incorrect assumption on the
directory structure of the virtualenv directory. According to the
virtualenv docs only POSIX systems follow the structure
"/path/to/venv/bin/", while windows systems use "/path/to/venv/Scripts"
for the location of the binary files, most importantly including the
python interpreter (see:
https://virtualenv.pypa.io/en/stable/userguide/#windows-notes).

This meant that the wrong path was added to `exec-path` and so we were
starting the python shell installed on the system, rather than the one
bundled inside of the virtualenv.

This should be addressed in the attached patch file - it just adds a
special case for Windows and prepends that path to the system PATH.

I also updated the docs for the `python-shell-calculate-exec-path`
function to make it more clear that using the python interpreter bundled
in the virtualenv will automatically set the PYTHONPATH to point at the
libraries inside of the virtualenv - something that was not immediately
obvious to me.



Steps to verify that patch is working correctly:
  1. (On Windows)
  2. Create virtualenv (eg. 'virtualenv ~/venv')
  3. Install a library NOT currently installed on the your system inside
  the virtualenv. ex:
       ~/venv/Scripts/activate.bat
       pip install sphinx
       deactivate.bat
  4. Start emacs
  5. Set `python-shell-virtualenv-root` to the root directory of your
  virtualenv (eg. "/home/jtimmons/venv")
  6. M-x run-python
  7. Make sure that you can import the library installed in the
  virtualenv in step 3 (eg. 'import sphinx')