Use web3.eth.generateGasPrice for gas price

[iamdefinitelyahuman]

What I did

Allow use of custom gas strategies via the web3 gas price API

How I did it

• When determining gas price, use web3.eth.generateGasPrice if no value is given in the config.
• Set the default strategy to check eth_gasPrice

How to verify it

Run tests.

[codecov-commenter]

Codecov Report

Merging #530 into master will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #530   +/-   ##
=======================================
  Coverage   88.44%   88.44%           
=======================================
  Files          56       56           
  Lines        6247     6249    +2     
  Branches     1380     1380           
=======================================
+ Hits         5525     5527    +2     
  Misses        493      493           
  Partials      229      229           

Impacted Files	Coverage Δ
brownie/network/account.py	94.11% <100.00%> (ø)
brownie/network/web3.py	97.19% <100.00%> (+0.05%)	⬆️

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[michwill]

Is it possible to choose between different gas strategies? E.g. fast/medium/slow/glacier in web3py