py/objmodule: Add support for __dict__.

This matches class `__dict__`, and is similarly gated on
MICROPY_CPYTHON_COMPAT. Unlike class though, because modules's globals are
actually dict instances, the result is a mutable dictionary.

This work was funded through GitHub Sponsors.

Signed-off-by: Jim Mussared <jim.mussared@gmail.com>

tests/import/module_dict.py

@@ -0,0 +1,17 @@
+# test __dict__ attribute of a user module
+
+import sys
+
+if not hasattr(sys, "__dict__"):
+    print("SKIP")
+    raise SystemExit
+
+
+import import1b
+
+# dict of a user module (read/write)
+print(import1b.var)
+print(import1b.__dict__["var"])
+import1b.__dict__["var"] = "hello"
+print(import1b.var)
+print(import1b.__dict__["var"])