Remove stale requirements

[AndrewC19]

Removed dowhy requriement that is not used in this repo

[codecov]

Codecov Report

Merging #62 (bc6d1bb) into main (ff34ff9) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main      #62   +/-   ##
=======================================
  Coverage   88.83%   88.83%           
=======================================
  Files          12       12           
  Lines         788      788           
=======================================
  Hits          700      700           
  Misses         88       88           

Impacted Files	Coverage Δ
...al_testing/generation/abstract_causal_test_case.py	98.79% <ø> (ø)
causal_testing/testing/causal_test_engine.py	95.23% <ø> (ø)
causal_testing/specification/causal_dag.py	98.40% <100.00%> (ø)
causal_testing/specification/scenario.py	78.00% <100.00%> (ø)
causal_testing/specification/variable.py	84.84% <100.00%> (ø)
causal_testing/testing/causal_test_case.py	100.00% <100.00%> (ø)
causal_testing/testing/causal_test_outcome.py	78.68% <100.00%> (ø)
causal_testing/testing/estimators.py	81.71% <100.00%> (ø)
... and 4 more

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