docs/library: Warn that ustruct doesn't handle spaces in format strings.

And also add a test to capture the CPython difference.

docs/library/ustruct.rst

@@ -12,6 +12,11 @@ Supported format codes: ``b``, ``B``, ``h``, ``H``, ``i``, ``I``, ``l``,
 ``L``, ``q``, ``Q``, ``s``, ``P``, ``f``, ``d`` (the latter 2 depending
 on the floating-point support).
 
+.. admonition:: Difference to CPython
+   :class: attention
+
+   Whitespace is not supported in format strings.
+
 Functions
 ---------
 

tests/cpydiff/modules_struct_whitespace_in_format.py

@@ -0,0 +1,13 @@
+"""
+categories: Modules,struct
+description: Struct pack with whitespace in format, whitespace ignored by CPython, error on uPy
+cause: MicroPython is optimised for code size.
+workaround: Don't use spaces in format strings.
+"""
+import struct
+
+try:
+    print(struct.pack('b b', 1, 2))
+    print('Should have worked')
+except:
+    print('struct.error')