Add completion for pipx. (#1719)

* Add completion for pipx.

* Add pipx  completion to clean_files.txt.

* Format pipx.completion.bash to match the project's coding style.

* Format pipx.completion.bash with ./lint_clean_files.sh.

completion/available/pipx.completion.bash

@@ -0,0 +1,6 @@
+#!/usr/bin/env bash
+# pipx completion
+
+if _command_exists register-python-argcomplete && _command_exists pipx; then
+	eval "$(register-python-argcomplete pipx)"
+fi