lang/python: add support for more env managers

+ Rewritten +conda support
+ Adds +pyenv and +pyvenv flags with support.
+ New +ipython flag to enable ipython REPL support
+ Added pipenv support. This is the new default, instead of pyenv, and
  isn't hidden behind a module flag because it is officially endorsed by
  python.

Addresses #736

modules/lang/python/autoload/python.el

@@ -5,3 +5,25 @@
   "Open the Python REPL."
   (interactive)
   (process-buffer (run-python nil t t)))
+
+;;;###autoload
+(defun +python-version ()
+  "Return the currently installed version of python on your system or active in
+the current pipenv.
+
+This is not necessarily aware of env management tools like virtualenv, pyenv or
+pipenv, unless those tools have modified the PATH that Emacs picked up when you
+started it."
+  (let* ((command (if (pipenv-project-p)
+                      "pipenv run python --version"
+                    "python --version"))
+         (bin (car (split-string command " "))))
+    (unless (executable-find bin)
+      (user-error "Couldn't find %s executable in PATH" bin))
+    (with-temp-buffer
+      (let ((p (apply #'call-process bin nil (current-buffer) nil
+                      (cdr (split-string command " " t))))
+            (output (string-trim (buffer-string))))
+        (unless (zerop p)
+          (user-error "'%s' failed: %s" command output))
+        (nth 1 (split-string output " " t))))))