check-files.py: use class fields for class-wide constants
check-files.py: use class fields for class-wide constants

In an issue tracker, heading and files_exemptions are class-wide
constants, so make them so instead of being per-instance fields.

NXPmicro/mbedtls,Mbed-TLS/mbedtls,ARMmbed/mbedtls,NXPmicro/mbedtls,NXPmicro/mbedtls,Mbed-TLS/mbedtls,Mbed-TLS/mbedtls,ARMmbed/mbedtls,ARMmbed/mbedtls,ARMmbed/mbedtls,NXPmicro/mbedtls,Mbed-TLS/mbedtls