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.

Author: nicoddemus

src/_pytest/config/__init__.py

@@ -511,9 +511,10 @@ def _importconftest(self, conftestpath):
         # 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:
-            return self._conftestpath2mod[key]
-        except KeyError:
+        mod = self._conftestpath2mod.get(key)
+        if mod is not None:
+            return mod
+        else:
             pkgpath = conftestpath.pypkgpath()
             if pkgpath is None:
                 _ensure_removed_sysmodule(conftestpath.purebasename)
@@ -525,8 +526,8 @@ def _importconftest(self, conftestpath):
                     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