pip: Fix uninitialized variable during check_mode (#4379)

During check_mode (`--check`), the variable change could be
used uninitialized, yielding this error:

`UnboundLocalError: local variable 'changed' referenced before assignment`

This changeset simply initializes it to False.

packaging/language/pip.py

@@ -410,6 +410,7 @@ def main():
             out += out_pip
             err += err_pip
 
+            changed = False
             if name:
                 for pkg in name:
                     is_present = _is_present(pkg, version, out.split())