Fix TypeError with MultiDictProxy and Python 3.8

[bdraco]

What do these changes do?

Fix non-subscriptable MultiDictProxy error as seen in #1105 when using older versions of multidict

Are there changes in behavior for the user?

no

Related issue number

fixes #1105

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.00%. Comparing base (bfef130) to head (542f7ca).
Report is 307 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1107   +/-   ##
=======================================
  Coverage   95.00%   95.00%           
=======================================
  Files          30       30           
  Lines        4448     4448           
  Branches      391      391           
=======================================
  Hits         4226     4226           
  Misses        196      196           
  Partials       26       26           

Flag	Coverage Δ
CI-GHA	94.96% <100.00%> (ø)
MyPy	39.95% <100.00%> (ø)
OS-Linux	99.35% <100.00%> (ø)
OS-Windows	99.45% <100.00%> (ø)
OS-macOS	99.04% <100.00%> (ø)
Py-3.10.11	98.92% <100.00%> (ø)
Py-3.10.14	99.18% <100.00%> (ø)
Py-3.11.9	99.18% <100.00%> (ø)
Py-3.12.5	99.18% <100.00%> (ø)
Py-3.13.0-rc.1	99.18% <100.00%> (ø)
Py-3.8.10	98.87% <100.00%> (ø)
Py-3.8.18	99.12% <100.00%> (ø)
Py-3.9.13	98.87% <100.00%> (ø)
Py-3.9.19	99.12% <100.00%> (ø)
Py-pypy7.3.11	99.18% <100.00%> (ø)
Py-pypy7.3.16	99.17% <100.00%> (ø)
Py-pypy7.3.17	99.21% <100.00%> (ø)
VM-macos-latest	99.04% <100.00%> (ø)
VM-ubuntu-latest	99.35% <100.00%> (ø)
VM-windows-latest	99.45% <100.00%> (ø)
pytest	99.35% <100.00%> (ø)

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[bdraco]

Tested locally, confirmed fixed