Import lazy_fixture explicitly due to pytest namespace deprecation

[jklaise]

pytest has deprecated their namespace hooks so functionality from plugins must be imported explicitly. This bug was found by running mypy with the latest pytest in the environment here: #280 .

[codecov]

Codecov Report

Merging #281 into master will decrease coverage by 1.96%.
The diff coverage is 96.25%.

@@            Coverage Diff             @@
##           master     #281      +/-   ##
==========================================
- Coverage   85.24%   83.28%   -1.97%     
==========================================
  Files          55       55              
  Lines        6684     6510     -174     
==========================================
- Hits         5698     5422     -276     
- Misses        986     1088     +102     

Impacted Files	Coverage Δ
alibi/explainers/integrated_gradients.py	90.85% <ø> (+0.49%)	⬆️
...libi/explainers/tests/test_integrated_gradients.py	100.00% <ø> (ø)
alibi/explainers/tests/utils.py	92.00% <ø> (-0.18%)	⬇️
alibi/explainers/tests/test_anchor_tabular.py	26.78% <50.00%> (-45.67%)	⬇️
alibi/explainers/tests/conftest.py	84.09% <95.45%> (-7.94%)	⬇️
alibi/explainers/cem.py	79.42% <100.00%> (ø)
alibi/explainers/cfproto.py	76.40% <100.00%> (ø)
alibi/explainers/counterfactual.py	81.46% <100.00%> (ø)
alibi/explainers/tests/test_ale.py	100.00% <100.00%> (ø)
alibi/explainers/tests/test_anchor_base.py	100.00% <100.00%> (ø)
... and 13 more