Update to GNAT Community 2020

[jklmnn]

When compiling SXML with GNAT Community 2020 I get the following error:

sxml.adb:66:13: error: not fully conformant with declaration at sxml.ads:320
sxml.adb:66:13: error: predicate of "D" does not match

[jklmnn]

It seems that the type Content_Base_Type and its subtype Content_Type need to have the same predicate.

[senier]

It seems that the type Content_Base_Type and its subtype Content_Type need to have the same predicate.

No, this is an issue in the code that remained undetected in previous compiler versions. While the spec uses Content_Base_Type, the implementation uses Content_Type which is clearly an error.