Adding curve25519 to tinyec
Tinyec only supports Weierstrass curves, with an equation of the form $y^2 = x^3 + a x + b$. Curve25519 is a Montgomery curve, with an equation of the form $B y^2 = x^3 + A x^2 + x$. Tinyec's arithmetic can't do Curve25519 since its $A$ parameter is nonzero.
It would be less work to adapt Scapy-ssl_tls to one of the existing interfaces to Curve25519 than to write Montgomery curve arithmetic yourself.