py-jaraco.context: add missing dependency to py-backports.tarfile for python < 3.12

[someuser12]

Description

py-jaraco.context needs py-backports.tarfile for Python < 3.12 (see https://github.com/jaraco/jaraco.context/blob/main/pyproject.toml), otherwise it fails with a message like:

File /opt/local/Library/Frameworks/Python.framework/Versions/3.10/lib/python3.10/site-packages/jaraco/context/__init__.py, line 19, in <module>
from backports import tarfile
ModuleNotFoundError: No module named 'backports'

This PR adds a port for py-backports.tarfile and explicits the dependency of py-jaraco.context to it.

Type(s)

• [x ] bugfix
• enhancement
• security fix

Tested on

macOS 11.7.10 20G1427 x86_64
Xcode 12.4 12D4e

Verification

Have you

• followed our Commit Message Guidelines?
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• checked that there aren't other open pull requests for the same change?
• referenced existing tickets on Trac with full URL in commit message?
• checked your Portfile with port lint?
• tried existing tests with sudo port test?
• tried a full install with sudo port -vst install?
• tested basic functionality of all binary files?
• checked that the Portfile's most important variants haven't been broken?

[macportsbot]

Notifying maintainers:
@reneeotten for port py-jaraco.context.

[reneeotten]

thanks for the fix @someuser12 ! Please address the one review comment and I'll happily merge this.