Pin setuptools_scm to <8

There are some breaking changes, and this is making it so that the
`_version.py` file generated is not compatible with Python 2 anymore.
I don't see an obvious or easy way to figure out what version of
`setuptools_scm` is in use, so we'll just pin the version for now.

I recommend removing `setuptools_scm` entirely in the future.

changelog.d/1346.bugfix.rst

@@ -0,0 +1 @@
+Pinned ``setuptools_scm`` to ``<8``, which should make the generated ``_version.py`` file compatible with all supported versions of Python.

pyproject.toml

@@ -3,7 +3,7 @@ requires = [
     "setuptools; python_version != '3.3'",
     "setuptools<40.0; python_version == '3.3'",
     "wheel",
-    "setuptools_scm"
+    "setuptools_scm<8.0"
 ]
 build-backend = "setuptools.build_meta"