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

PR Summary

PR Checklist

• Has pytest style unit tests (and pytest passes).
• Is Flake 8 compliant (run flake8 on changed files to check).
• New features are documented, with examples if plot related.
• Documentation is sphinx and numpydoc compliant (the docs should build without error).
• Conforms to Matplotlib style conventions (install flake8-docstrings and run flake8 --docstring-convention=all).
• New features have an entry in doc/users/next_whats_new/ (follow instructions in README.rst there).
• API changes documented in doc/api/next_api_changes/ (follow instructions in README.rst there).