Upgrade to use latest pyrlp and py-evm package about sharding

[hwwhww]

Ouch, I figured out the package dependencies hell...

• sharding package requirement:
  • the latest version of py-evm
  • the rlp>=1.0.1
• sharding testing requirement:
  • Vyper with commit 044d1565df370cd31c00fc7fb728672647f39cf2 (on 9 Mar)
    • In this version, Vyper requires ethereum==2.1.3
  • eth-tester[py-evm]
    • If PyEthereum21 (including ethereum==2.1.3) is installed, it will crash because (i) py-evm using rlp==1.0.1 and (ii) pyethereum 2.1.3 using rlp<1.

Solution - Modify Vyper

1. In the latest Vyper codebase, the dependancy of pyethereum is replaced with locked py-evm version. But that still not good for sharding because we need newer py-evm.
   1. Option 1: The only things that import py-evm in Vyper are just some constants: https://github.com/ethereum/vyper/blob/0d92d86752bcfca875e042cec6048488db3d479c/vyper/parser/parser_utils.py#L3 which could be easily duplicated in Vyper and make them decoupled in package level.
   2. Option 2: evm.constants are unlikely to be changed. Just unlock the depandency from py-evm==0.2.0a12 to py-evm.