Use importlib.metadata for single version source (microsoft#124)

Removes the hardcoded version from `__version__.py` and reads it from
package metadata instead, per the [Python Packaging
Guide](https://packaging.python.org/en/latest/discussions/versioning/).
Only `pyproject.toml` needs to be updated at release time.

If the package isn't installed, `PackageNotFoundError` propagates with a
clear message.

---------

Co-authored-by: tpellissier <tpellissier@microsoft.com>
Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: 3 people

CONTRIBUTING.md

@@ -119,6 +119,6 @@ published release:
 
 ```bash
 # After publishing v0.1.0b4, bump to v0.1.0b5 on main
-# Update both pyproject.toml and src/PowerPlatform/Dataverse/__version__.py
+# Update version in pyproject.toml
 # Commit directly to main: "Bump version to 0.1.0b5 for next development cycle"
 ```

src/PowerPlatform/Dataverse/__init__.py

@@ -1,6 +1,8 @@
 # Copyright (c) Microsoft Corporation.
 # Licensed under the MIT license.
 
-from .__version__ import __version__
+from importlib.metadata import version
+
+__version__ = version("PowerPlatform-Dataverse-Client")
 
 __all__ = ["__version__"]

src/PowerPlatform/Dataverse/__version__.py

@@ -35,7 +35,7 @@
     VALIDATION_UNSUPPORTED_CACHE_KIND,
 )
 
-from ..__version__ import __version__ as _SDK_VERSION
+from .. import __version__ as _SDK_VERSION
 
 _USER_AGENT = f"DataverseSvcPythonClient:{_SDK_VERSION}"
 _GUID_RE = re.compile(r"[0-9a-fA-F]{8}-[0-9a-fA-F]{4}-[0-9a-fA-F]{4}-[0-9a-fA-F]{4}-[0-9a-fA-F]{12}")