Add an AnyIO sprint (#1433)

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Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

Author: agronholm

src/content/sprints/anyio.md

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+---
+title: "AnyIO"
+numberOfPeople: "2-8"
+pythonLevel: "Intermediate"
+contactPerson:
+  name: "Alex Grönholm"
+  email: "[email protected]"
+  github: "agronholm"
+links:
+  - title: "GitHub repo"
+    url: "https://github.com/agronholm/anyio"
+  - title: "Gitter chat"
+    url: "https://matrix.to/#/#python-trio_AnyIO:gitter.im"
+---
+
+We'll mostly work on improving the documentation, in particular adding guides
+for migrating existing asyncio or Trio apps and libraries to the AnyIO API. If
+you have a pet peeve with AnyIO, or would just like to ask questions or get help
+with your AnyIO migration, feel free to join!