fix: virtual price rounding error

[AlbertoCentonze]

In tweak_price the calculation of the virtual price can be off by 1 due a combination of isqrt + division rounding down.

In particular this line would revert:

twocrypto-ng/contracts/main/Twocrypto.vy

Line 918 in 8621f90

assert virtual_price > old_virtual_price # dev: virtual price decreased

Because of this line lacking a + 1 previously:

twocrypto-ng/contracts/main/Twocrypto.vy

Lines 906 to 907 in 8621f90

	xcp: uint256 = isqrt(xp[0] * xp[1])
	virtual_price = 10**18 * xcp // total_supply + 1

While this issue doesn't seem to have cause issues onchain (probably it doesn't happen when pool is used as intended), this still create issues in testing and could potentially prevent people from withdrawing liquidity/exchanging. For this reason we increase the value of virtual_price by 1e-18 that can be considered a safe negligible amount that can hardly be misused given virtual price has 18 decimals of precision.

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This is part 2 of 3 in a stack made with GitButler:

•  3  fix: onchain reverts #20
•  2  fix: virtual price rounding error #19 👈
•  1  chore: bump Vyper version to 0.4.0 #18