pip completions: use builtin source rather than eval function.

share/completions/pip.fish

@@ -1,4 +1,4 @@
 if command -sq pip
-    eval (pip completion --fish)
+    pip completion --fish | source
 end
 

share/completions/pip2.fish

@@ -1,3 +1,3 @@
 if command -sq pip2
-    eval (pip2 completion --fish)
+    pip2 completion --fish | source
 end

share/completions/pip3.fish

@@ -1,3 +1,3 @@
 if command -sq pip3
-    eval (pip3 completion --fish)
+    pip3 completion --fish | source
 end

share/completions/pipenv.fish

@@ -1,3 +1,3 @@
 if command -sq pipenv
-    eval (pipenv --completion)
+    pipenv --completion | source
 end