Don't generate PDF/A-1b with object streams

Acrobat insists that PDF/A-1b should not have object streams.
Other programs like veraPDF disagree with this restriction, but
we can accommodate Acrobat so we will.

Also add more tests around this.

tests/test_pdfa.py

@@ -0,0 +1,34 @@
+# © 2021 James R. Barlow: github.com/jbarlow83
+#
+# This Source Code Form is subject to the terms of the Mozilla Public
+# License, v. 2.0. If a copy of the MPL was not distributed with this
+# file, You can obtain one at http://mozilla.org/MPL/2.0/.
+
+import pikepdf
+import pytest
+
+check_ocrmypdf = pytest.helpers.check_ocrmypdf
+
+
+@pytest.mark.parametrize('optimize', (0, 3))
+@pytest.mark.parametrize('pdfa_level', (1, 2, 3))
+def test_pdfa(resources, outpdf, optimize, pdfa_level):
+    check_ocrmypdf(
+        resources / 'francais.pdf',
+        outpdf,
+        '--plugin',
+        'tests/plugins/tesseract_noop.py',
+        f'--output-type=pdfa-{pdfa_level}',
+        f'--optimize={optimize}',
+    )
+    if pdfa_level in (2, 3):
+        # PDF/A-2 allows ObjStm
+        assert b'/ObjStm' in outpdf.read_bytes()
+    elif pdfa_level == 1:
+        # PDF/A-1 might allow ObjStm, but Acrobat does not approve it, so
+        # we don't use it
+        assert b'/ObjStm' not in outpdf.read_bytes()
+
+    with pikepdf.open(outpdf) as pdf:
+        with pdf.open_metadata() as m:
+            assert m.pdfa_status == f'{pdfa_level}B'