gnu: python-aiohttp: Propagate attrs.

...as required by its egginfo.

* gnu/packages/python-web.scm (python-aiohttp)[propagated-inputs]: Add
PYTHON-ATTRS.

gnu/packages/python-web.scm

@@ -292,6 +292,7 @@ for adding, removing and dropping callbacks.")
     (propagated-inputs
      (list python-aiodns
            python-aiosignal
+           python-attrs
            python-async-timeout
            python-charset-normalizer
            python-frozenlist