[logging] Allow LogRecord.exc_info to be False

[tjkuson]

Logger._log only normalises exc_info when it is truthy, so logger.info("msg", exc_info=False) creates a record whose exc_info is False. The standard library itself passes exc_info=False in asyncio, for example.

Given that the this value can technically be any falsy, the type could arguably be typing.Any (or just object). But I think this would make the annotation unhelpful for the vast majority of uses, and just adding Literal[False] to the union captures the most common way to pass a falsy value.

[github-actions]

Diff from mypy_primer, showing the effect of this PR on open source code:

trio (https://github.com/python-trio/trio)
+ src/trio/_tests/test_highlevel_serve_listeners.py:148: error: Value of type "tuple[type[BaseException], BaseException, TracebackType | None] | tuple[None, None, None] | Literal[False]" is not indexable  [index]
+ src/trio/_tests/test_highlevel_serve_listeners.py:149: error: Value of type "tuple[type[BaseException], BaseException, TracebackType | None] | tuple[None, None, None] | Literal[False]" is not indexable  [index]
+ src/trio/_core/_tests/test_instrumentation.py:209: error: "Literal[False]" object is not iterable  [misc]

zulip (https://github.com/zulip/zulip)
+ zerver/tests/test_email_mirror.py:2232: error: Value of type "tuple[type[BaseException], BaseException, TracebackType | None] | tuple[None, None, None] | Literal[False]" is not indexable  [index]

sphinx (https://github.com/sphinx-doc/sphinx)
+ sphinx/util/logging.py: note: In member "filter" of class "_RaiseOnWarningFilter":
+ sphinx/util/logging.py:411:47: error: Value of type "tuple[type[BaseException], BaseException, TracebackType | None] | tuple[None, None, None] | Literal[False]" is not indexable  [index]

scrapy (https://github.com/scrapy/scrapy)
+ tests/test_pipeline_files.py:390: error: Value of type "tuple[type[BaseException], BaseException, TracebackType | None] | tuple[None, None, None] | Literal[False]" is not indexable  [index]
+ tests/test_request_cb_kwargs.py:182: error: Value of type "tuple[type[BaseException], BaseException, TracebackType | None] | tuple[None, None, None] | Literal[False]" is not indexable  [index]
+ tests/test_request_cb_kwargs.py:183: error: Value of type "tuple[type[BaseException], BaseException, TracebackType | None] | tuple[None, None, None] | Literal[False]" is not indexable  [index]
+ tests/test_request_cb_kwargs.py:188: error: Value of type "tuple[type[BaseException], BaseException, TracebackType | None] | tuple[None, None, None] | Literal[False]" is not indexable  [index]
+ tests/test_request_cb_kwargs.py:189: error: Value of type "tuple[type[BaseException], BaseException, TracebackType | None] | tuple[None, None, None] | Literal[False]" is not indexable  [index]
+ tests/test_crawl.py:209: error: Value of type "tuple[type[BaseException], BaseException, TracebackType | None] | tuple[None, None, None] | Literal[False]" is not indexable  [index]
+ tests/test_crawl.py:222: error: Value of type "tuple[type[BaseException], BaseException, TracebackType | None] | tuple[None, None, None] | Literal[False]" is not indexable  [index]

[srittau]

Since we don't allow arbitrary falsy values to be passed to log() etc., I think Literal[False] is the correct annotation. This will also be a hint to users that falsely use is not None (and there are a few according to primer) that something is amiss.

[srittau]

I've also opened python/cpython#158839 and python/cpython#158840 to normalize falsy values before passing them to LogRecord to match the documentation (which says that LogRecord.exc_info can be an exception (info) or None).