Add overloads to MockRouter.__call__

[flaeppe]

• Resolves an [union-attr] error shown when calling MockRouter methods

Repro:

import respx
mocker = respx.mock()
mocker.get(path="http://localhost/")

Results in an error:

error: Item "function" of "Union[MockRouter, Callable[..., Any]]" has no attribute "get"  [union-attr]

[codecov-commenter]

Codecov Report

Merging #187 (6491c5c) into master (8511566) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master      #187   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           20        20           
  Lines         2712      2716    +4     
  Branches       289       289           
=========================================
+ Hits          2712      2716    +4     

Impacted Files	Coverage Δ
respx/router.py	100.00% <100.00%> (ø)

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[lundberg]

Sweet! This will please mypy 👍