Pytest fixes

[iamdefinitelyahuman]

What I did

• Bugfix: script imports from the stdlib
• Add project path to sys.path prior to loading pytest
• Improve readability of exception when compilation fails while running pytest

How to verify it

Run tests.

[codecov-commenter]

Codecov Report

Merging #556 into master will decrease coverage by 0.00%.
The diff coverage is 69.23%.

@@            Coverage Diff             @@
##           master     #556      +/-   ##
==========================================
- Coverage   88.00%   88.00%   -0.01%     
==========================================
  Files          56       56              
  Lines        6312     6319       +7     
  Branches     1396     1396              
==========================================
+ Hits         5555     5561       +6     
- Misses        522      523       +1     
  Partials      235      235              

Impacted Files	Coverage Δ
brownie/project/scripts.py	87.14% <66.66%> (ø)
brownie/test/plugin.py	76.31% <66.66%> (-4.94%)	⬇️
brownie/_cli/test.py	85.71% <100.00%> (+0.71%)	⬆️
brownie/network/state.py	93.04% <0.00%> (+1.73%)	⬆️

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