[backport] CI fixes (#6933)

* Relax shotgun test. (#6900)

It's non-deterministic algorithm, the test is flaky.

* Disable pylint error. (#6911)

* [CI] Skip external memory gtest on osx. (#6901)

* [CI] Fix custom metric test with empty dataset. (#6917)

* Reduce Travis environment setup time. (#6912)

* Remove unused r from travis.
* Don't update homebrew.
* Don't install indirect/unused dependencies like libgit2, wget, openssl.
* Move graphviz installation to conda.

* Relax shotgun test. (#6918)

* Relax test for decision stump in distributed environment. (#6919)

* Backport cupy fix.

python-package/xgboost/core.py

@@ -1872,7 +1872,9 @@ class Booster(object):
                 )
             )
             return _prediction_output(shape, dims, preds, False)
-        if lazy_isinstance(data, "cupy.core.core", "ndarray"):
+        if lazy_isinstance(data, "cupy.core.core", "ndarray") or lazy_isinstance(
+            data, "cupy._core.core", "ndarray"
+        ):
             from .data import _transform_cupy_array
             data = _transform_cupy_array(data)
             interface = data.__cuda_array_interface__
@@ -2027,7 +2029,7 @@ class Booster(object):
         """
         if isinstance(fout, (STRING_TYPES, os.PathLike)):
             fout = os.fspath(os.path.expanduser(fout))
-            fout = open(fout, 'w')
+            fout = open(fout, 'w')  # pylint: disable=consider-using-with
             need_close = True
         else:
             need_close = False