Add cancel_by_tag support

[irstmon]

Add cancel_by_tag support

What

Adds a new cancel_by_tag(tag) method to PushoverAPI, implementing the
[cancel_by_tag receipts endpoint](https://pushover.net/api/receipts#cancel-by-tag)
of the Pushover API.

Why

The Pushover API allows attaching arbitrary tags to emergency-priority messages
via the tags parameter. These tags can be used to cancel all matching receipts
at once - without needing to store individual receipt IDs. This is useful for
applications like monitoring systems that may send many related emergency
notifications (e.g. per-server alerts) and want to cancel them all at once when
an issue is resolved.

Usage

p = PushoverAPI("your_app_token")

# Send emergency message with a tag (tags parameter is passed through as-is)
p.send_message(
    "user_token",
    "Server mail01 is down",
    priority=2,
    retry=60,
    expire=3600,
    tags="mail01,rack-a,chicago",
)

# Later: cancel all receipts tagged "chicago"
p.cancel_by_tag("chicago")

Changes

src/pushover_complete/pushover_api.py

Added cancel_by_tag(tag) method after cancel_receipt(). It follows the
exact same pattern as the existing cancel_receipt() method - a single call
to _generic_post with the tag as the URL parameter:

POST https://api.pushover.net/1/receipts/cancel_by_tag/{tag}.json

tests/PushoverAPI/test_PushoverAPI.py

Added two new tests after the existing receipt cancel tests:

• test_PushoverAPI_cancels_by_tag - verifies a successful cancel call
• test_PushoverAPI_raises_error_on_bad_tag_cancel - verifies that a
  BadAPIRequestError is raised when the tag has no active receipts

The new tests import receipt_cancel_by_tag_callback from responses_callbacks
and TEST_TAG from constants, following the exact same structure as the
existing receipt cancel tests.

tests/responses_callbacks.py

Added receipt_cancel_by_tag_callback to mock the
/receipts/cancel_by_tag/{tag}.json endpoint. It validates the app token and
checks that the tag in the URL matches TEST_TAG, returning a 400 error
otherwise. Also added TEST_TAG to the imports from tests.constants.

tests/constants.py

Added one new constant:

TEST_TAG = "server-down"