fix: Don't install COPYING at top level of wheel (#105)

Telling Poetry to install the `COPYING` file without qualification
causes it to install that file at the very top level of the wheel,
outside the `shortuuid/` directory, and thus to potentially clash with
other packages.

We only need to explicitly install it in the sdist.  The wheel still
ends up with a copy of the file in `shortuuid-*.dist-info/COPYING`, so
no information is lost.

pyproject.toml

@@ -16,7 +16,9 @@ classifiers = [
 homepage = "https://github.com/skorokithakis/shortuuid/"
 authors = ["Stavros Korokithakis <[email protected]>"]
 readme = "README.md"
-include = ["COPYING"]
+include = [
+  { path = "COPYING", format = "sdist" }
+]
 
 [tool.poetry.scripts]
 shortuuid = "shortuuid.cli:cli"