[pip* completions] Silence stderr

For some reason, these tools print an upgrade message to stderr, even
when stdout goes somewhere else.

share/completions/pip.fish

@@ -1,4 +1,4 @@
 if command -sq pip
-    pip completion --fish | source
+    pip completion --fish 2>/dev/null | source
 end
 

share/completions/pip2.fish

@@ -3,5 +3,5 @@ if command -sq pip2
     # See discussion at https://github.com/fish-shell/fish-shell/pull/4448
     # and pip bug at https://github.com/pypa/pip/pull/4755
     # Keep this even after pip fix is upstreamed for users not on the latest pip
-    pip2 completion --fish | string replace -r -- " -c\s+pip\b" " -c pip2" | source
+    pip2 completion --fish 2>/dev/null | string replace -r -- " -c\s+pip\b" " -c pip2" | source
 end

share/completions/pip3.fish

@@ -3,5 +3,5 @@ if command -sq pip3
     # See discussion at https://github.com/fish-shell/fish-shell/pull/4448
     # and pip bug at https://github.com/pypa/pip/pull/4755
     # Keep this even after pip fix is upstreamed for users not on the latest pip
-    pip3 completion --fish | string replace -r -- " -c\s+pip\b" " -c pip3" | source
+    pip3 completion --fish 2>/dev/null | string replace -r -- " -c\s+pip\b" " -c pip3" | source
 end

share/completions/pipenv.fish

@@ -1,3 +1,3 @@
 if command -sq pipenv
-    pipenv --completion | source
+    pipenv --completion 2>/dev/null | source
 end