Add backports.lzma to requirements

Signed-off-by: Tomas Hozza <thozza@redhat.com>
Add extra version variable and get method

Signed-off-by: Tomas Hozza <thozza@redhat.com>
Add SpecFile.set_extra_version() draft

Signed-off-by: Tomas Hozza <thozza@redhat.com>
Add basic test for set_extra_version()

Signed-off-by: Tomas Hozza <thozza@redhat.com>