Drop support for python 3.6

[lundberg]

Since httpx no longer supports python 3.6, neither respx would need to.

[codecov-commenter]

Codecov Report

Base: 100.00% // Head: 100.00% // No change to project coverage 👍

Coverage data is based on head (53f42d4) compared to base (8d9e149).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff            @@
##            master      #218   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           20        20           
  Lines         2796      2744   -52     
  Branches       350       350           
=========================================
- Hits          2796      2744   -52     

Impacted Files	Coverage Δ
respx/handlers.py	100.00% <ø> (ø)
respx/models.py	100.00% <ø> (ø)
tests/test_mock.py	100.00% <ø> (ø)
tests/test_router.py	100.00% <ø> (ø)
tests/test_stats.py	100.00% <ø> (ø)
tests/test_transports.py	100.00% <ø> (ø)
respx/router.py	100.00% <100.00%> (ø)
tests/test_api.py	100.00% <100.00%> (ø)

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