Fix resolver garbage collection during pending queries

[bdraco]

What do these changes do?

Converts _callback from a @staticmethod to an instance method so that the callback holds a strong reference to the resolver instance. This prevents the resolver from being garbage collected while queries are pending.

Are there changes in behavior for the user?

Previously, queries on temporary resolvers would fail:

# Failed with DNSError(24, 'DNS query cancelled')
await DNSResolver().query("example.com", "A")

Now this pattern works correctly because the future's callback keeps the resolver alive until the query completes.

Related issue number

Fixes #209

Checklist

• I think the code is well written
• Unit tests for the changes exist
• Documentation reflects the changes

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.69%. Comparing base (e088cf4) to head (a14c046).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #211      +/-   ##
==========================================
+ Coverage   97.31%   97.69%   +0.37%     
==========================================
  Files           3        3              
  Lines         559      564       +5     
  Branches       38       38              
==========================================
+ Hits          544      551       +7     
+ Misses          8        7       -1     
+ Partials        7        6       -1     

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