mail/sympa: update 6.2.54 -> 6.2.56, fix security issue

• A vulnerability has been discovered in Sympa web interface by which attacker can execute arbitrary code with root privileges.

PR: 246701
Submitted by: William F. Dudley Jr. <wfdudley@gmail.com>
Approved by: dgeo@centrale-marseille.fr (maintainer)
MFH: 2020Q2
Relnotes: https://github.com/sympa-community/sympa/releases/tag/6.2.56
Security: CVE-2020-10936

		https://sympa-community.github.io/security/2020-002.html
		https://github.com/sympa-community/sympa/issues/943