feat!: require Python 3.15 and fix unsigned session cookies

BREAKING CHANGE: the minimum supported Python version is now 3.15.

Python 3.15:

- Raise requires-python from >=3.13 to >=3.15.
- Replace the 3.13 trove classifier with 3.15.
- Pin the toolchain via a .python-version file.
- Pin setup-uv to Python 3.15 in the publish workflow.
- Correct the stale "Python 3.7.0 or higher" claim in the README.
- Verified against CPython 3.15.0; all extras resolve to 3.15 wheels.

Session cookie fixes:

- Fix save_session() crashing with AttributeError on every request.
- The str sid was decoded as bytes when SESSION_USE_SIGNER is off.
- That is the default, so it broke the stock configuration.
- Only decode in the signer branch, where sign() actually returns bytes.
- Handle a missing secret_key instead of calling sign() on None.
- A missing signer now warns and skips the cookie, like open_session().
- Both bugs predate this release and affect all Python 3 versions.

Tests:

- Add a pytest suite; the project previously had no tests at all.
- Cover the cookie round-trip for both signed and unsigned sessions.
- Cover session state surviving a follow-up request.
- Cover the missing secret_key path.
- Three of the five tests fail without the save_session() fix.
- Add pytest and pytest-asyncio to the dev dependency group.
- Enable asyncio_mode = "auto" so tests need no per-test markers.

Version:

- Bump the package version to 4.0.0.
- Derive __version__ from installed package metadata.
- It was hardcoded and had drifted to 3.0.0 while the release was 3.0.2.
- pyproject.toml is now the single source of truth for the version.
- Commit uv.lock to pin the development environment.

pyproject.toml

@@ -1,9 +1,9 @@
 [project]
 name = "quart-session"
-version = "3.0.2"
+version = "4.0.0"
 description = "Server-side sessions for Quart"
 readme = "README.md"
-requires-python = ">=3.13"
+requires-python = ">=3.15"
 license = "BSD-3-Clause"
 license-files = ["LICENSE"]
 authors = [{ name = "Kroket Ltd.", email = "code@kroket.io" }]
@@ -15,7 +15,7 @@ classifiers = [
     "Intended Audience :: Developers",
     "Operating System :: OS Independent",
     "Programming Language :: Python :: 3",
-    "Programming Language :: Python :: 3.13",
+    "Programming Language :: Python :: 3.15",
     "Topic :: Internet :: WWW/HTTP :: Dynamic Content",
     "Topic :: Software Development :: Libraries :: Python Modules",
 ]
@@ -34,6 +34,8 @@ redis-trio = ["trio>=0.22"]
 [dependency-groups]
 dev = [
     "build>=1.3.0",
+    "pytest>=8.0",
+    "pytest-asyncio>=0.24",
 ]
 
 [build-system]
@@ -42,3 +44,7 @@ build-backend = "setuptools.build_meta"
 
 [tool.setuptools.packages.find]
 include = ["quart_session*"]
+
+[tool.pytest.ini_options]
+testpaths = ["tests"]
+asyncio_mode = "auto"