Use PEP 639 SPDX license identifier in pyproject.toml (#226)

Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: thodson-usgs

pyproject.toml

@@ -8,7 +8,8 @@ description = "Discover and retrieve water data from U.S. federal hydrologic web
 readme = "README.md"
 requires-python = ">=3.8"
 keywords = ["USGS", "water data"]
-license = {file = "LICENSE.md"}
+license = "CC0-1.0"
+license-files = ["LICENSE.md"]
 authors = [
   {name = "Timothy Hodson", email = "thodson@usgs.gov"},
 ]