mpy-cross: Fix source file name in file-not-found error.

Found by Ruff with F821.

Signed-off-by: Angus Gratton <angus@redyak.com.au>

mpy-cross/mpy_cross/__init__.py

@@ -100,7 +100,7 @@ def compile(src, dest=None, src_path=None, opt=None, march=None, mpy_cross=None,
     if not src:
         raise ValueError("src is required")
     if not os.path.exists(src):
-        raise CrossCompileError("Input .py file not found: {}.".format(src_py))
+        raise CrossCompileError("Input .py file not found: {}.".format(src))
 
     args = []