Make vendored symposion into an installable Python package

This allows us to install with `pip install "-e vendor/symposion"` similar to
the other vendored packages. There's no good reason for this to be different to
the others and depend on PYTHONPATH hacking.

Re-add

pinaxcon/settings.py

@@ -16,7 +16,6 @@ PROJECT_ROOT = os.path.abspath(os.path.join(os.path.dirname(__file__), os.pardir
 PACKAGE_ROOT = os.path.abspath(os.path.dirname(__file__))
 DJANGO_ROOT = os.path.abspath(os.path.dirname(django.__file__))
 BASE_DIR = PACKAGE_ROOT
-sys.path.append(os.path.join(PROJECT_ROOT, 'vendor'))
 
 
 ### USER SETTINGS

vendor/symposion/pyproject.toml

@@ -0,0 +1,3 @@
+[build-system]
+requires = ["setuptools>=42"]
+build-backend = "setuptools.build_meta"

vendor/symposion/setup.cfg

@@ -0,0 +1,3 @@
+[metadata]
+name = symposion
+version = attr: symposion.__version__

vendor/symposion/setup.py

@@ -0,0 +1,3 @@
+import setuptools
+
+setuptools.setup()