Fix controller proxies after the client is garbage collected

[RazerM]

The proxy stored weakref.proxy(client), but nothing else kept the
client alive, so the one-expression idiom
SpaceTrackClient(...).basicspacedata.gp(...) collected the client
mid-expression and raised ReferenceError, and hasattr/getattr on a
proxy that outlived its client raised ReferenceError instead of
reporting the attribute missing. The client-to-proxy reference cycle
this creates is handled by the garbage collector, and the finalizer
still fires because it does not reference the instance.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.29%. Comparing base (168de31) to head (51e72ae).
⚠️ Report is 15 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #167      +/-   ##
==========================================
+ Coverage   96.33%   98.29%   +1.96%     
==========================================
  Files           7        7              
  Lines        1147     1293     +146     
  Branches      129      135       +6     
==========================================
+ Hits         1105     1271     +166     
+ Misses         25       12      -13     
+ Partials       17       10       -7     

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