doc: fix readthedocs version

Use unshallow clone [1] to negate the default readthedocs clone with
depth 50.

1. readthedocs/readthedocs.org#9708 (comment)

Closes #255

Author: DifferentialOrange

.readthedocs.yaml

@@ -0,0 +1,8 @@
+version: 2
+build:
+  os: "ubuntu-20.04"
+  tools:
+    python: "3.10"
+  jobs:
+    post_checkout:
+      - git fetch --unshallow

CHANGELOG.md

@@ -202,6 +202,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - Allow any MessagePack supported type as a request key (#240).
 - Puting test files in pip package (#238).
 - Make connection close idempotent (#250).
+- readthedocs version (#255).
 
 ## 0.9.0 - 2022-06-20