incorrectly reported overflow exception

[AlbertoCentonze]

Version Information

• vyper Version (output of vyper --version): 0.4.0

What's your issue about?

@external
def foo() -> uint256:
    return (10**18)**2

panics at compilation with the following error, which is clearly wrong given that 10**36 clearly fits in a uint256:

vyper.exceptions.OverflowException: Base is too large, calculation will always overflow

  contract "contracts/token.vy:3", function "foo", line 3:12 
       2 def foo() -> uint256:
  ---> 3     return (10**18)**2
  -------------------^