Update devDependencies

* @types/express                    ^4.17.6  →   ^4.17.8
* @types/express-serve-static-core  ^4.17.7  →  ^4.17.12
* mocha                              ^8.0.1  →    ^8.1.3
* typescript                         ^3.9.5  →    ^4.0.2
* xo                                ^0.32.0  →   ^0.33.1

Author: XhmikosR