Support creating PDFs conforming to PDF/A-1a, PDF/A-2a and PDF/A-3a

[malte-laukoetter]

Weasyprint currently does not support creating files conforming to the a-conformance-levels of PDF/A. This conformance level indicates that the files are additionally to the requirements of the u-variant (or b-variant for PDF/A-1) also is accessible. Weasyprint already supports creating PDF/UA-1 conforming (and therefore accessible) files.

Could you add support for the a-conformance-levels as well?

In my testing I was able to create PDF/A-2a conforming files by choosing PDF/A-2u and also using the --pdf-tags flag to create an accessible pdf file. The only PDF/A-2a rule the file was then not conforming to was that the conformance-level is not indicated in the metadata (https://github.com/veraPDF/veraPDF-validation-profiles/wiki/PDFA-Parts-2-and-3-rules#rule-664-3) By manually overwriting the metadata afterwards I was able to create a PDF/A-2a conforming file.

Therefore the following addition to pdfa.py should be enough to fix this:

'pdf/a-1a': (
    partial(pdfa, version=1, variant='A'),
    {'version': '1.4', 'identifier': True, 'srgb': True, 'pdf_tags': True}),
'pdf/a-2a': (
    partial(pdfa, version=2, variant='A'),
    {'version': '1.7', 'identifier': True, 'srgb': True, 'pdf_tags': True}),
'pdf/a-3a': (
    partial(pdfa, version=3, variant='A'),
    {'version': '1.7', 'identifier': True, 'srgb': True, 'pdf_tags': True}),