Fix eldoc for custom python-shell-interpreter

[Pitometsu]

With

'(python-shell-interpreter "ipython")

There's error:

eldoc error: (file-error Searching for program No such file or directory ipython)

In *Messages*.

[pwalsh]

@Pitometsu

If python-shell-interpreter is "ipython", it seems unlikely that you are in a virtual environment being managed by Pipenv, as if it were, there would be a value like /Users/netsu/.local/share/virtualenvs/csr-44kfYsMA/bin/python.

Did you add "ipython" manually? Maybe, as I wrote here you want pipenv.el to use an ipython executable as the python-shell-interpreter from the active virtual environment, if such an executable exists?

[Pitometsu]

It would be nice, if pipenv remember original (customized by user) python-shell-interpreter value, and would try to call it from environment. But proposed approach ok for me.

[pwalsh]

pipenv.el no longer modifies python-shell-interpreter, so user settings are respected for this variable.

pipenv.el now pushes the virtual env path to (exec-path) when a pipenv-managed project is activated, for example by (pipenv-activate).

• https://github.com/pwalsh/pipenv.el/blob/master/pipenv.el#L163
• https://github.com/pwalsh/pipenv.el/blob/master/pipenv.el#L327
• https://github.com/pwalsh/pipenv.el/blob/master/test/pipenv-test.el#L31