setup.py: use importlib.resources from python 3.7 to find core.sr.ht Makefile

importlib.resources will guarantee to find any importable module, and
check that the resource ('Makefile') can be found within it.

This lets us get rid of the current getsitepackages() hack and
environment variable fallback, which is both verbose and badly handles
some edge cases when installing modules, and instead use the recommended
way to find data files shipped with a python module.

setup.py

@@ -2,31 +2,11 @@
 from distutils.core import setup
 import subprocess
 import os
-import site
 import sys
+import importlib.resources
 
-if hasattr(site, 'getsitepackages'):
-    pkg_dirs = site.getsitepackages()
-    if site.getusersitepackages():
-        pkg_dirs.append(site.getusersitepackages())
-    for pkg_dir in pkg_dirs:
-        srht_path = os.path.join(pkg_dir, "srht")
-        if os.path.isdir(srht_path):
-            break
-    else:
-        raise Exception("Can't find core srht module in your site packages "
-            "directories. Please install it first.")
-else:
-    srht_path = os.getenv("SRHT_PATH")
-    if not srht_path:
-        raise Exception("You're running inside a virtual environment. "
-            "Due to virtualenv limitations, you need to set the "
-            "$SRHT_PATH environment variable to the path of the "
-            "core srht module.")
-    elif not os.path.isdir(srht_path):
-        raise Exception(
-            "The $SRHT_PATH environment variable points to an invalid "
-            "directory: {}".format(srht_path))
+with importlib.resources.path('srht', 'Makefile') as f:
+    srht_path = f.parent.as_posix()
 
 make = os.environ.get("MAKE", "make")
 subp = subprocess.run([make, "SRHT_PATH=" + srht_path])