feat(pip{,3}): add fallback 3rd-party completion loader

https://github.com/Bash-it/bash-it/blob/e38696a0acfdb6e4fbeb6963801c417d6ca7e9a7/completion/available/pip.completion.bash

completions/.gitignore

@@ -211,6 +211,7 @@
 /pinfo
 /ping4
 /ping6
+/_pip3
 /pkg_deinstall
 /pkg_info
 /pkgconf

completions/Makefile.am

@@ -316,6 +316,7 @@ bashcomp_DATA = 2to3 \
 		pidof \
 		pine \
 		ping \
+		_pip \
 		_pipenv \
 		pkg-config \
 		pkg-get \
@@ -751,6 +752,7 @@ CLEANFILES = \
 	pinfo \
 	ping4 \
 	ping6 \
+	_pip3 \
 	pkg_deinstall \
 	pkg_info \
 	pkgconf \
@@ -1126,6 +1128,8 @@ symlinks: $(DATA)
 		alpine
 	$(ss) ping \
 		ping4 ping6
+	$(ss) _pip \
+		_pip3
 	$(ss) _pipenv \
 		_black _blackd _flask _httpx
 	$(ss) pkg-config \

completions/_pip

@@ -0,0 +1,9 @@
+# 3rd party completion loader for commands emitting        -*- shell-script -*-
+# their completion using "$cmd completion --bash".  For example, pip uses this
+# form of dynamic completions.
+#
+# This serves as a fallback in case the completion is not installed otherwise.
+
+eval -- "$("$1" completion --bash 2>/dev/null)"
+
+# ex: filetype=sh

test/fallback/completions/Makefile.am

@@ -24,6 +24,7 @@ EXTRA_DIST = \
 	newgrp \
 	nmcli \
 	nox \
+	pip \
 	pipenv \
 	renice \
 	repomanage \

test/fallback/completions/pip

@@ -0,0 +1 @@
+../../../completions/_pip