remove obsolete, risky forms

As emacs.py isn't distributed any more by Emacs, don't to keep forms relying on it.
Exception displayed points to source, not to the temporary file executed internally

`py-output-buffer' made customizable, default follows `py-shell-name'.

In python-mode.el v6 series output of
`py-execute-...'-commands arrives in buffer created by
`py-shell'. It's name is composed WRT to Python version
used, it's path etc.

v5 used a single buffer, "*Python Output*", this
bevavior is still provided by command
`py-execute-python-mode-v5'

Now, choose the name of output-buffer freely,
also permitting a function which composes it, using date
and time for example.

New boolean `py-enforce-output-buffer-p' makes sure
`py-output-buffer' set by `defcustom' or since is used,
not guessed from environment. Default is nil.

`py-execute-file-base' returns result as string, not the marker at point-max

`py-execute-file-base', when `py-verbose-p', message the file user wants to execute, not the temp-file, executed indeed

- boolean `py-store-result-p'
  When non-nil, put resulting string of `py-execute-...' into kill-ring, so it might be yanked. Default is nil
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