xmlsec 1.3.3 incompatability

[ayakael]

xmlsec 1.3.3 removed a bunch of deprecated functions that python-xmlsec uses. The following PR removed those deprecated functions:
lsh123/xmlsec#720

[deronnax]

and the build is currently failing because of the removal of xmlSecCryptoAppKeyLoad, that lsh123/xmlsec#720 did remove.

[ayakael]

@deronnax This PR addresses these changes #280

[sergei-maertens]

FYI, since the end of January Arch linux ships 1.3.3 in their repositories, I was just bitten by this