The filename ends with .py for Python syntax highlighting

SellToken.vy.py

import ERC20 as ERC20

owner: public(address)
tokenAddress: public(address)
erc20: ERC20

@public
def __init__(_tokenAddress: address):
    self.owner = msg.sender
    self.tokenAddress = _tokenAddress
    self.erc20 = ERC20(self.tokenAddress)

@payable
@public
def buyToken():
    assert msg.value == 10 ** 16
    assert_modifiable(self.erc20.transfer(msg.sender, 500))

@public
def endSale():
    assert msg.sender == self.owner
    selfdestruct(self.owner)