David,

Let's inherit the current python-marshmallow to avoid all this 
duplication:

+(define-public python-marshmallow-3.2
+  (package
+    (inherit python-marshmallow)
+    (version "3.2.1")
+    (source
+     (origin
+       (method url-fetch)
+       (uri (pypi-uri "marshmallow" version))
+       (sha256
+        (base32 
"1w18klwj0z9bqxj252qpj1hz8l46nl27sbc89rkajc7mln73wbws"))))))

If you're thinking, ‘wait a minute, what happens to my precious 
3.2 package if someone updates 3.9 in a weird way?’, well, you're 
right.  There are trade-offs to both.

Kind regards,

T G-R