Remove one of the tracebacks from conftest import failures

This removes the KeyError from the traceback chain when an
conftest fails to import:

    return self._conftestpath2mod[key]
    E   KeyError: WindowsPath('D:/projects/pytest/.tmp/root/foo/conftest.py')

    During handling of the above exception, another exception occurred:
    ...
        raise RuntimeError("some error")
    E   RuntimeError: some error

    During handling of the above exception, another exception occurred:
    ...
    E   _pytest.config.ConftestImportFailure: (...)

By slightly changing the code, we can remove the first chain, which is often
very confusing to users and doesn't help with anything.

Fix #7223

src/_pytest/config/__init__.py

@@ -1,5 +1,6 @@
 """ command line options, ini-file and conftest.py processing. """
 import argparse
+import contextlib
 import copy
 import enum
 import inspect
@@ -511,12 +512,14 @@ class PytestPluginManager(PluginManager):
         # Using Path().resolve() is better than py.path.realpath because
         # it resolves to the correct path/drive in case-insensitive file systems (#5792)
         key = Path(str(conftestpath)).resolve()
-        try:
+
+        with contextlib.suppress(KeyError):
             return self._conftestpath2mod[key]
-        except KeyError:
+
         pkgpath = conftestpath.pypkgpath()
         if pkgpath is None:
             _ensure_removed_sysmodule(conftestpath.purebasename)
+
         try:
             mod = conftestpath.pyimport()
             if (
@@ -525,8 +528,8 @@ class PytestPluginManager(PluginManager):
                 and not self._using_pyargs
             ):
                 _fail_on_non_top_pytest_plugins(conftestpath, self._confcutdir)
-            except Exception:
-                raise ConftestImportFailure(conftestpath, sys.exc_info())
+        except Exception as e:
+            raise ConftestImportFailure(conftestpath, sys.exc_info()) from e
 
         self._conftest_plugins.add(mod)
         self._conftestpath2mod[key] = mod