Access pdf annotations while inside pikepdf.Pdf context manager.

Depending on the exact way the pdf file is written, iterating over
Annots can fail after the file has been closed.  This is not the case
for matplotlib-generated pdfs, but is the case for mplcairo-generated
ones.  As a simple repro of the different behavior between
"in-the-contextmanager" and "out-of-the-contextmanager":
```
from matplotlib import pyplot as plt
plt.figtext(.5, .5, "hello, world", url="https://www.google.com")
plt.savefig("/tmp/test.pdf", backend="pdf")
import pikepdf
with pikepdf.Pdf.open("/tmp/test.pdf") as pdf:
    page = pdf.pages[0]
    print(repr(page.Annots)) # within contextmanager: ok
with pikepdf.Pdf.open("/tmp/test.pdf") as pdf:
    page = pdf.pages[0]
print(repr(page.Annots))  # after contextmanager: AttributeError
```

Author: anntzer

lib/matplotlib/tests/test_backend_pdf.py

@@ -227,13 +227,15 @@ def test_text_urls():
         with pikepdf.Pdf.open(fd) as pdf:
             annots = pdf.pages[0].Annots
 
-    for y, fragment in [('0.1', 'plain'), ('0.4', 'mathtext')]:
-        annot = next(
-            (a for a in annots if a.A.URI == f'{test_url}{fragment}'),
-            None)
-        assert annot is not None
-        # Positions in points (72 per inch.)
-        assert annot.Rect[1] == decimal.Decimal(y) * 72
+            # Iteration over Annots must occur within the context manager,
+            # otherwise it may fail depending on the pdf structure.
+            for y, fragment in [('0.1', 'plain'), ('0.4', 'mathtext')]:
+                annot = next(
+                    (a for a in annots if a.A.URI == f'{test_url}{fragment}'),
+                    None)
+                assert annot is not None
+                # Positions in points (72 per inch.)
+                assert annot.Rect[1] == decimal.Decimal(y) * 72
 
 
 @needs_usetex
@@ -251,12 +253,14 @@ def test_text_urls_tex():
         with pikepdf.Pdf.open(fd) as pdf:
             annots = pdf.pages[0].Annots
 
-    annot = next(
-        (a for a in annots if a.A.URI == f'{test_url}tex'),
-        None)
-    assert annot is not None
-    # Positions in points (72 per inch.)
-    assert annot.Rect[1] == decimal.Decimal('0.7') * 72
+            # Iteration over Annots must occur within the context manager,
+            # otherwise it may fail depending on the pdf structure.
+            annot = next(
+                (a for a in annots if a.A.URI == f'{test_url}tex'),
+                None)
+            assert annot is not None
+            # Positions in points (72 per inch.)
+            assert annot.Rect[1] == decimal.Decimal('0.7') * 72
 
 
 def test_pdfpages_fspath():