Add a typing-compatible mechanism for ad-hoc attributes on various objects

pytest has several instances where plugins set their own attributes on
objects they receive in hooks, like nodes and config. Since plugins are
detached from these object's definition by design, this causes a problem
for type checking because these attributes are not defined and mypy
complains.

Fix this by giving these objects a "store" which can be used by plugins
in a type-safe manner.

Currently this mechanism is private. We can consider exposing it at a
later point.

src/_pytest/pastebin.py

@@ -1,7 +1,12 @@
 """ submit failure or test session information to a pastebin service. """
 import tempfile
+from typing import IO
 
 import pytest
+from _pytest.store import StoreKey
+
+
+pastebinfile_key = StoreKey[IO[bytes]]()
 
 
 def pytest_addoption(parser):
@@ -26,25 +31,26 @@ def pytest_configure(config):
         # when using pytest-xdist, for example
         if tr is not None:
             # pastebin file will be utf-8 encoded binary file
-            config._pastebinfile = tempfile.TemporaryFile("w+b")
+            config._store[pastebinfile_key] = tempfile.TemporaryFile("w+b")
             oldwrite = tr._tw.write
 
             def tee_write(s, **kwargs):
                 oldwrite(s, **kwargs)
                 if isinstance(s, str):
                     s = s.encode("utf-8")
-                config._pastebinfile.write(s)
+                config._store[pastebinfile_key].write(s)
 
             tr._tw.write = tee_write
 
 
 def pytest_unconfigure(config):
-    if hasattr(config, "_pastebinfile"):
+    if pastebinfile_key in config._store:
+        pastebinfile = config._store[pastebinfile_key]
         # get terminal contents and delete file
-        config._pastebinfile.seek(0)
-        sessionlog = config._pastebinfile.read()
-        config._pastebinfile.close()
-        del config._pastebinfile
+        pastebinfile.seek(0)
+        sessionlog = pastebinfile.read()
+        pastebinfile.close()
+        del config._store[pastebinfile_key]
         # undo our patching in the terminal reporter
         tr = config.pluginmanager.getplugin("terminalreporter")
         del tr._tw.__dict__["write"]