Change default cache-control response to 'no-store'

[ahupp]

The default cache-control header returned by sse-starlete is
"no-cache", which (confusingly!) does not actually mean "don't cache",
but instead that caching is allowed but the result must be
re-validated on each fetch. From MDN:

The no-cache response directive indicates that the response can be
stored in caches, but the response must be validated with the origin
server before each reuse, even when the cache is disconnected from
the origin server.

This is probably harmless as long as there's not an ETag or
Last-Modified header that would allow revalidation, but it does make
it much easier to unexpectedly enable caching or globally audit cache
behavior.

This PR:

• Changes the default to "no-store", which does disallow caching

• Uses starlette's MutableHeaders to build the headers; this
  normalizes to all lower-case so we're independent of case if an
  explicit cache-control is passed in.

• Adds a test for header handling.

• Adds ruff to the dev dependencies and updates make lint to
  resolve a deprecation warning in the command.