Add httpx dependency

[mike0sv]

from 0.21 starlette needs httpx for test client
encode/starlette#652

[codecov]

Codecov Report

Base: 60.01% // Head: 85.40% // Increases project coverage by +25.39% 🎉

Coverage data is based on head (65b766b) compared to base (6b511fd).
Patch coverage: 91.40% of modified lines in pull request are covered.

Additional details and impacted files

@@             Coverage Diff             @@
##             main     #490       +/-   ##
===========================================
+ Coverage   60.01%   85.40%   +25.39%     
===========================================
  Files          95       96        +1     
  Lines        8135     8255      +120     
===========================================
+ Hits         4882     7050     +2168     
+ Misses       3253     1205     -2048     

Impacted Files	Coverage Δ
mlem/cli/main.py	84.58% <ø> (+27.66%)	⬆️
mlem/contrib/catboost.py	98.33% <ø> (+43.10%)	⬆️
mlem/contrib/tensorflow.py	96.33% <ø> (+42.69%)	⬆️
mlem/core/import_objects.py	98.07% <ø> (+26.92%)	⬆️
mlem/core/model.py	94.59% <ø> (+21.62%)	⬆️
mlem/contrib/rabbitmq.py	51.80% <50.00%> (ø)
mlem/contrib/sklearn.py	84.78% <66.66%> (+27.42%)	⬆️
mlem/runtime/server.py	85.29% <78.57%> (-5.19%)	⬇️
mlem/runtime/interface.py	84.61% <84.61%> (+28.36%)	⬆️
mlem/api/commands.py	80.31% <100.00%> (+46.71%)	⬆️
... and 85 more

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