rtutil: Add RTDateTime class.

See comments for rationale.

rtutil: Fix crash when multiple RTLinkCaches race.

This was most likely to happen with multiple bean-checks running in
parallel: they would both see a new RT URL, fetch it, and try to
insert it into the cache. That's not a problem as long as they're
both inserting the same URL, so catch that exception.

rtutil: Avoid loading from a URI in RTLinkCache.setup.

See comments for rationale. RT#10543.

rtutil: Add debug logging to RTLinkCache.setup.

To help with RT#10543.

rtutil: Add RTUtil.metadata_regexp() classmethod.

The accruals check script wants to be able to search RT links in
all kinds of metadata, not just rt-id as the filter currently
handles.

rtutil: Explicit fspath cast to avoid a Py3.6 typecheck issue.

On Py3.6, either sqlite3.connect() doesn't take a path-like object,
or the type stubs don't know that.

setup: Disallow untyped defs.

Mostly this meant giving annotations to low-value functions like
the error classes and __init_subclass__, but it's worth it for
the future strictness+documentation value.

setup: Enable stricter type checking.

This caught the "return instead of raise" bug in meta_project.

rtutil: Add RTLinkCache class to cache links to disk.

This will greatly reduce RT requests across multiple runs
and speed up link checking/conversion.

rtutil: Make RT.parse a classmethod.

Because it works, and I can imagine it being useful to have this
method more handy to other code.

rtutil: Add RT.parse method.

This method can parse the ticket and (optional) attachment IDs
out of our short-hand link formats, both for Ledger and Beancount.

rtutil: Start module.

For now, this is basically the Python version of
ledger-tag-convert.plx. It knows how to create RT web links from
ticket and attachment IDs. It confirms that those objects actually
exist too. It may grow to encompass other functionality in the
future.