Fix #352: support callable and PathLike entries in default_config_files

[bw2]

Summary

• Fixes handling configuration files without conventional paths #352 by allowing entries in default_config_files to be os.PathLike objects or zero-argument callables that return an open file-like object, in addition to glob path strings.
• Callables are invoked each time the parser opens config files, the returned stream is closed by the parser after parsing, and exceptions raised during the call are wrapped in a ConfigFileParserException with the original cause preserved.
• Each callable entry gets a unique source label so distinct sources do not collapse together in format_values() output, even when multiple callables share a __name__ (e.g. lambdas) or return streams with identical .name attributes.
• Validates default_config_files entries up front in __init__ and updates format_help() so help text renders a readable label for each entry (path string for path-likes, function name for callables).

Test plan

• python -m unittest tests.test_configargparse
• black . --check
• pydoctor ./configargparse.py --intersphinx=https://docs.python.org/3/objects.inv --docformat=google -W

[Copilot]

Pull request overview

This PR extends ArgParser(default_config_files=...) to support non-string sources by allowing os.PathLike entries and zero-argument callables that return open file-like objects, addressing #352’s need to source config from non-filesystem locations.

Changes:

• Add upfront validation and runtime support for default_config_files entries that are os.PathLike or callables returning streams, including unique per-entry source labels and guaranteed stream cleanup.
• Improve robustness of config-source tracking/output: prevent format_values() from breaking on | in source names and avoid source-key collisions for multiple callables.
• Update help/error-message behavior and add extensive unit coverage for callable/pathlike default config sources and stream-closing guarantees.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
configargparse.py	Implements callable/PathLike default_config_files, unique source labels, safer cleanup, and help/values formatting updates.
tests/test_configargparse.py	Adds comprehensive tests for callable/PathLike default config sources, error wrapping, and stream lifecycle behavior; updates regex expectations for error strings.

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