Use resources loader to handle non-filesystem situations (#120)

* Use a resources loader if possible

* Remove logic duplication

certifi/core.py

@@ -6,25 +6,40 @@
 
 This module returns the installation location of cacert.pem or its contents.
 """
+import importlib
 import os
 
 try:
-    from importlib.resources import read_text
+    import importlib.resources
+    # Defeat lazy module importers.
+    importlib.resources.open_binary
+    _HAVE_RESOURCE_READER = True
 except ImportError:
-    # This fallback will work for Python versions prior to 3.7 that lack the
-    # importlib.resources module but relies on the existing `where` function
-    # so won't address issues with environments like PyOxidizer that don't set
-    # __file__ on modules.
-    def read_text(_module, _path, encoding="ascii"):
-        with open(where(), "r", encoding=encoding) as data:
-            return data.read()
+    _HAVE_RESOURCE_READER = False
 
+try:
+    import pkg_resources
+    # Defeat lazy module importers.
+    _HAVE_PKG_RESOURCES = True
+except ImportError:
+    _HAVE_PKG_RESOURCES = False
 
-def where():
-    f = os.path.dirname(__file__)
+_PACKAGE_NAME = "certifi"
+_CACERT_NAME = "cacert.pem"
 
-    return os.path.join(f, "cacert.pem")
+
+def where():
+    if _HAVE_PKG_RESOURCES:
+        return pkg_resources.resource_filename(_PACKAGE_NAME, _CACERT_NAME)
+    else:
+        mod = importlib.import_module(_PACKAGE_NAME)
+        path = os.path.dirname(mod.__file__)
+        return os.path.join(path, _CACERT_NAME)
 
 
 def contents():
-    return read_text("certifi", "cacert.pem", encoding="ascii")
+    if _HAVE_RESOURCE_READER:
+        return importlib.resources.read_text(_PACKAGE_NAME, _CACERT_NAME, encoding="ascii")
+    else:
+        with open(where(), "r", encoding="ascii") as data:
+            return data.read()