fix: prevent RuntimeError in asyncio.cluster.NodesManager.initialize()

[jakob-keller]

Pull Request check-list

• Do tests and lints pass with this change?
• Do the CI tests pass with this change (enable it first in your forked repo and wait for the github action build to finish)?
• Is the new or changed code fully tested?
• Is a documentation update included (if this change modifies existing APIs, or introduces new ones)?
• Is there an example added to the examples folder (if applicable)?

Description of change

The change prevents RuntimeError being raised when valkey.asyncio.cluster.NodeManager.startup_nodes are modified while performing .initialize().

Fixes #181

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 76.25%. Comparing base (ca5c7c5) to head (6dfe091).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #184      +/-   ##
==========================================
- Coverage   76.26%   76.25%   -0.01%     
==========================================
  Files         130      130              
  Lines       33970    33970              
==========================================
- Hits        25906    25904       -2     
- Misses       8064     8066       +2     

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