Mark version as dynamic in pyproject.toml

We need to set `dynamic = ["version"]` in `pyproject.toml`, since the
spec requires that required fields must be listed in `dynamic` if
they're to be provided by the build backend (maturin for us).

See PyO3/maturin#2390 for the maturin issue
to adhere to the spec.

pyproject.toml

@@ -10,6 +10,7 @@ classifiers = [
     "Programming Language :: Python :: Implementation :: CPython",
     "Programming Language :: Python :: Implementation :: PyPy",
 ]
+dynamic = ["version"]
 
 
 [tool.maturin]