Prepare setup.py/.cfg for additional import-time dependencies

Reading the version number using attr: rsrcfork.__version__ will no
longer work properly if rsrcfork has non-stdlib dependencies at import
time, because setuptools needs to be able to import rsrcfork and read
the version number before the dependencies are installed.

As a workaround, our setup.py now manually parses the version number
from rsrcfork/__init__.py using the ast module.

setup.py

@@ -1,5 +1,39 @@
 #!/usr/bin/env python3
 
+import ast
 import setuptools
 
-setuptools.setup()
+
+def attr(file, name):
+	"""Read the constant value of a global variable from a Python file without importing/executing it.
+	
+	The variable in question must be assigned a constant literal
+	(as understood by :func:`ast.literal_eval`)
+	in a simple assignment.
+	The variable *should* only be assigned once
+	(later assignments are silently ignored).
+	
+	Based on https://github.com/pypa/setuptools/issues/1960#issue-547330414.
+	"""
+	
+	with open(file, "rb") as f:
+		module = ast.parse(f.read())
+	
+	for node in ast.iter_child_nodes(module):
+		if (
+			isinstance(node, ast.Assign)
+			and len(node.targets) == 1
+			and isinstance(node.targets[0], ast.Name)
+			and node.targets[0].id == name
+		):
+			return ast.literal_eval(node.value)
+	else:
+		raise ValueError(f"No simple assignment of variable {name!r} found in {file!r}")
+
+
+setuptools.setup(
+	# Read the version number from the module source code without importing or executing it.
+	# This is necessary because at the time that setup.py is executed,
+	# the dependencies necessary to import rsrcfork may not be installed yet.
+	version=attr("rsrcfork/__init__.py", "__version__"),
+)