Do not raise on non-zero block height

[iamdefinitelyahuman]

What I did

Warn instead of raising an exception when block height is > 0 in development network - Closes #453

I also got distracted and added some intersphinx docs references for web3py

How I did it

• add brownie Warning classes, implement them in existing warnings
• warn instead of raising on block height > 0
• add documentation

How to verify it

Run tests.

[codecov-io]

Codecov Report

Merging #461 into master will decrease coverage by 2.83%.
The diff coverage is 75.00%.

@@            Coverage Diff             @@
##           master     #461      +/-   ##
==========================================
- Coverage   88.16%   85.32%   -2.84%     
==========================================
  Files          56       56              
  Lines        5981     5985       +4     
  Branches     1317     1316       -1     
==========================================
- Hits         5273     5107     -166     
- Misses        490      642     +152     
- Partials      218      236      +18     

Impacted Files	Coverage Δ
brownie/network/contract.py	90.08% <0.00%> (ø)
brownie/network/main.py	87.17% <66.66%> (+2.56%)	⬆️
brownie/exceptions.py	88.40% <100.00%> (+0.71%)	⬆️
brownie/test/managers/master.py	32.25% <0.00%> (-61.30%)	⬇️
brownie/test/__init__.py	42.10% <0.00%> (-57.90%)	⬇️
brownie/test/managers/runner.py	51.57% <0.00%> (-34.22%)	⬇️
brownie/test/fixtures.py	59.74% <0.00%> (-32.47%)	⬇️
brownie/test/managers/base.py	61.70% <0.00%> (-18.09%)	⬇️
brownie/test/plugin.py	65.62% <0.00%> (-15.63%)	⬇️
brownie/test/coverage.py	86.66% <0.00%> (-13.34%)	⬇️
... and 5 more

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