Drop 0.9-era date helper re-exports from the top-level webob package

[digitalresistor]

Drops the five date helpers that the top-level webob package has been
re-exporting for backwards compatibility since 0.9.7, and adds BaseRequest
to webob.__all__.

Background

parse_date, parse_date_delta, serialize_date, serialize_date_delta and
timedelta_to_seconds originally lived in webob/__init__.py. They moved to
webob.datetime_utils in 0196b09 (Nov 2009, version 0.9.7) and the imports
were left behind in __init__.py for compatibility. They were never
documented and never listed in webob.__all__. Since 2.0 is a major, this
seems like the moment to let them go — they remain available from
webob.datetime_utils.

The UTC tzinfo object and the year/month/week/day/hour/minute/
second timedelta constants are unaffected and remain importable from
webob; they're documented and used in docs/reference.txt.

Why now

This came out of #457. Once #477 lands and WebOb ships a py.typed marker,
names imported into __init__.py but absent from __all__ stop being
re-exports as far as type checkers are concerned. Installing #477 into a clean
venv and checking a consumer that does from webob import BaseRequest, parse_date:

# pyright, default settings — reportPrivateImportUsage is an error at basic level
error: "BaseRequest" is not exported from module "webob"
  Import from "webob.request" instead  [reportPrivateImportUsage]

# mypy --strict
error: Module "webob" does not explicitly export attribute "BaseRequest"  [attr-defined]

All six names fail the same way. BaseRequest is genuinely public and
documented, so it belongs in __all__; the five date helpers are better
dropped than blessed. With this change applied on top of #477, that consumer
file is clean under both pyright and mypy --strict.

Note this wouldn't have been caught by #477's own CI: mypy -p webob passes
clean there, because no_implicit_reexport flags the importing module and
nothing inside WebOb imports these from the top-level package. It only
surfaces for consumers, after release. A regression test belongs in the
tests/mypy/ harness that #477 introduces, so I've suggested it there rather
than adding a mypy dependency here.

Verification

• Full suite: 2392 passed, 1 xfailed, coverage still 100%
• from webob import parse_date now raises ImportError;
  webob.datetime_utils still exposes all five
• from webob import * still supports the documented
  res.cache_expires(2*day+4*hour) example
• flake8 clean without the # noqa: F401

See #457