fix: support desktop-selected Secret Service providers

[UglyEgg]

Problem

The package requires GNOME Keyring even though its runtime interface is the Freedesktop Secret Service API. That prevents desktop-selected providers from being treated as first-class implementations.

The current availability check also performs a real keyring operation. This can require a usable default collection, create one as a side effect, or prompt before the client actually needs a secret. Python Keyring opens a fresh Secret Service connection for each operation, so providers that authorize clients by D-Bus identity may prompt repeatedly.

Change

• Make GNOME Keyring a packaging suggestion rather than a hard Debian or RPM dependency.
• Validate Secret Service availability without reading or creating a secret.
• Prefer the Freedesktop default alias; if it is missing or stale, use the sole advertised collection, create a default only when no collection exists, and reject ambiguous multi-collection fallback.
• Reuse one serialized D-Bus connection for the backend lifetime so provider authorization can persist across operations. The backend does not cache plaintext and closes the connection explicitly or through a finalizer.

GNOME Keyring, KeePassXC, KWallet, and other conforming Secret Service implementations use the same code path. No provider-specific logic or secret-storage schema change is introduced.

Verification

• 10 focused tests cover non-invasive validation, default/sole/empty/ambiguous collection selection, connection reuse, explicit close/reopen, credential reads, writes, and deletion.
• Complete upstream suite: 31 passed.
• ruff check, Python bytecode compilation, and git diff --check pass.
• The equivalent downstream overlay has been exercised on Fedora 44 Plasma with KeePassXC as org.freedesktop.secrets, including cold start, login, sign-out/sign-in, reconnect, and one authorization prompt per client start.

Closes #3.

Development disclosure: I developed this patch with material assistance from OpenAI Codex under my direction, then reviewed, tested, and signed the final change.