Support for pytest 6 "--import-mode=importlib"

[youtux]

This is a continuation of the initial fix proposed in #381:

With the pytest 6 RC, a new --import-mode=importlib was added, which uses importlib rather than sys.path hacks to import test modules

This PR fixes the issue by using inspect.stack(1).frame.f_locals to access the dictionary of the locals of the caller module and modify them.

[codecov]

Codecov Report

Merging #384 into master will decrease coverage by 0.41%.
The diff coverage is 98.27%.

@@            Coverage Diff             @@
##           master     #384      +/-   ##
==========================================
- Coverage   95.54%   95.13%   -0.42%     
==========================================
  Files          49       49              
  Lines        1683     1705      +22     
  Branches      168      169       +1     
==========================================
+ Hits         1608     1622      +14     
- Misses         43       48       +5     
- Partials       32       35       +3     

Impacted Files	Coverage Δ
tests/steps/test_steps.py	100.00% <ø> (ø)
pytest_bdd/scenario.py	89.75% <92.30%> (-6.00%)	⬇️
pytest_bdd/steps.py	91.37% <100.00%> (+2.49%)	⬆️
pytest_bdd/utils.py	100.00% <100.00%> (ø)
tests/conftest.py	100.00% <100.00%> (ø)
tests/feature/test_scenario.py	100.00% <100.00%> (ø)
tests/feature/test_scenarios.py	100.00% <100.00%> (ø)
tests/utils.py	100.00% <100.00%> (ø)

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