vyper's range(start, start + N) reverts for negative numbers

Vyper is a pythonic Smart Contract Language for the Ethereum virtual machine. Starting in version 0.3.8 and prior to version 0.4.0b1, when looping over a range of the form range(start, start + N), if start is negative, the execution will always revert. This issue is caused by an incorrect assertion inserted by the code generation of the range stmt.parse_For_range(). The issue arises when start is signed, instead of using sle, le is used and start is interpreted as an unsigned integer for the comparison. If it is a negative number, its 255th bit is set to 1 and is hence interpreted as a very large unsigned integer making the assertion always fail. Any contract having a range(start, start + N) where start is a signed integer with the possibility for start to be negative is affected. If a call goes through the loop while supplying a negative start the execution will revert. Version 0.4.0b1 fixes the issue.