Migrate to pyproject.toml with setuptools_scm

Replace custom version management with setuptools_scm to eliminate
source file modifications during installation. Version is now derived
from git tags at build time without writing to tracked files.

Changes:
- Add pyproject.toml with full project metadata
- Simplify setup.py to minimal backwards-compatible shim
- Remove curateipsum/_version.py from version control
- Add _version.py to .gitignore (auto-generated at build time)
- Use PEP 440 compliant version strings

Fixes #18

curateipsum/_version.py

@@ -1 +0,0 @@
-version = "master"