Add PEP 518 pyproject.toml.

This is necessary to be able to install nMigen into a virtualenv that
does not have `wheel` installed in certain cases.

See #349.

pyproject.toml

@@ -0,0 +1,2 @@
+[build-system]
+requires = ["setuptools", "setuptools_scm", "wheel"]

setup.py

@@ -23,7 +23,6 @@ setup(
     #long_description="""TODO""",
     license="BSD",
     python_requires="~=3.6",
-    setup_requires=["setuptools", "setuptools_scm"],
     install_requires=[
         "importlib_metadata; python_version<'3.8'",  # for __version__ and nmigen._yosys
         "importlib_resources; python_version<'3.9'", # for nmigen._yosys