Revert "Add PEP 518 pyproject.toml."

This reverts commit 7fca037f9c.

This broke editable installs and has to be reverted due to a number
of pip issues:
  * pypa/pip#6375
  * pypa/pip#6434
  * pypa/pip#6438

We can put this back once PEP 517/518 support editable installs.
Until then the legacy behavior will suffice, and we should just teach
people to install the dependencies in virtualenvs or something...

pyproject.toml

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

setup.py

@@ -23,6 +23,7 @@ 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