Increase the upper boundary of the multidict dependency allowing v6

The `multidict` dependency used to be `multidict >=4.5, < 7.0` in `aiohttp` v3.8.1,
but was pinned down to `multidict >=4.5, < 6.0` in `aiohttp` v3.8.2
(#6550) in an attempt to help avoid
new errors.

However, https://github.com/python/bedevere/ uses `multidict==6.0.2`
so the requirements resolution fails with `aiohttp==3.8.2`:

```console
ERROR: Cannot install -r requirements.txt (line 1) and multidict==6.0.2 because these package versions have conflicting dependencies.

The conflict is caused by:
    The user requested multidict==6.0.2
    aiohttp 3.8.2 depends on multidict<6.0 and >=4.5

To fix this you could try to:
1. loosen the range of package versions you've specified
2. remove package versions to allow pip attempt to solve the dependency conflict
```

https://github.com/hugovk/bedevere/actions/runs/3095794573/jobs/5010600772

This patch makes it possible to have `multidict` v6 installed in the same env as `aiohttp` v3.8.3, as with `aiohttp` v3.8.1.

Partially related to closed #6600 and
merged #6550.

PR #6950

Co-Authored-By: Sviatoslav Sydorenko <[email protected]>

CHANGES/6950.bugfix

@@ -0,0 +1,7 @@
+Increased the upper boundary of the :doc:`multidict:index` dependency
+to allow for the version 6 -- by :user:`hugovk`.
+
+It used to be limited below version 7 in :doc:`aiohttp <index>` v3.8.1 but
+was lowered in v3.8.2 via :pr:`6550` and never brought back, causing
+problems with dependency pins when upgrading. :doc:`aiohttp <index>` v3.8.3
+fixes that by recovering the original boundary of ``< 7``.

aiohttp/web_ws.py

@@ -196,9 +196,9 @@ def _handshake(
         accept_val = base64.b64encode(
             hashlib.sha1(key.encode() + WS_KEY).digest()
         ).decode()
-        response_headers = CIMultiDict(  # type: ignore[var-annotated]
+        response_headers = CIMultiDict(
             {
-                hdrs.UPGRADE: "websocket",  # type: ignore[arg-type]
+                hdrs.UPGRADE: "websocket",
                 hdrs.CONNECTION: "upgrade",
                 hdrs.SEC_WEBSOCKET_ACCEPT: accept_val,
             }

setup.cfg

@@ -50,7 +50,7 @@ include_package_data = True
 install_requires =
   attrs >= 17.3.0
   charset-normalizer >=2.0, < 3.0
-  multidict >=4.5, < 6.0
+  multidict >=4.5, < 7.0
   async_timeout >= 4.0.0a3, < 5.0
   asynctest == 0.13.0; python_version<"3.8"
   yarl >= 1.0, < 2.0