Simplify prepare_command function and add deprecation warning

cibuildwheel/util.py

@@ -1,16 +1,21 @@
 from fnmatch import fnmatch
+import warnings
 
 
-def prepare_command(command, python, pip, project):
+def prepare_command(command, project):
     '''
-    Preprocesses a command by expanding variables like {python} or {pip}.
+    Preprocesses a command by expanding variables like {project}.
 
     For example, used for the before_build option, where the user would
     like to run a command like `python setup.py test`. If the command should run on
     Python 3, the user could write `{python} setup.py test`. This command would expand
     it out to python2 or python3 as appropriate.
     '''
-    return command.format(python=python, pip=pip, project=project)
+    if '{python}' in command or '{pip}' in command:
+        warnings.warn("'{python}' and '{pip}' are no longer needed, and have been deprecated. Simply use 'python' or 'pip' instead.",
+            DeprecationWarning)
+
+    return command.format(python='python', pip='pip', project=project)
 
 
 class BuildSkipper(object):