fix(python): HTTP request from pip-requirements-mode

pip-requirements-mode fetches the pypi.org package list via HTTP
request, which blocks. This can sometimes take unbearably long in cases
where the user has a slow or no internet connection.

This fix defers this behavior until the first time completion is
invoked.

Fix: #5998

modules/lang/python/config.el

@@ -311,6 +311,24 @@
   :after cython-mode)
 
 
+(use-package! pip-requirements
+  :defer t
+  :config
+  ;; HACK `pip-requirements-mode' performs a sudden HTTP request to
+  ;;   https://pypi.org/simple, which causes unexpected hangs (see #5998). This
+  ;;   advice defers this behavior until the first time completion is invoked.
+  ;; REVIEW More sensible behavior should be PRed upstream.
+  (defadvice! +python--init-completion-a (&rest args)
+    "Call `pip-requirements-fetch-packages' first time completion is invoked."
+    :before #'pip-requirements-complete-at-point
+    (unless pip-packages (pip-requirements-fetch-packages)))
+  (defadvice! +python--inhibit-pip-requirements-fetch-packages-a (fn &rest args)
+    "No-op `pip-requirements-fetch-packages', which can be expensive."
+    :around #'pip-requirements-mode
+    (letf ((#'pip-requirements-fetch-packages #'ignore))
+      (apply fn args))))
+
+
 ;;
 ;;; LSP