missing beakerx.button

With latest code on Jupyter Lab 1.2.9 with the standard labextension (see previous issue) I get the following error when doing a simple "from beakerx import *":

```python
from beakerx import *
from beakerx.object import beakerx
```

> ---------------------------------------------------------------------------
ModuleNotFoundError Traceback (most recent call last)
in
2 import pandas as pd
3
----> 4 from beakerx import *
5 from beakerx.object import beakerx

/usr/local/lib/python3.8/site-packages/beakerx/__init__.py in
14
15 from ._version import version_info, __version__
---> 16 from .commands import parse as beakerx_parse
17 from .easyform import *
18 from .spark import *

/usr/local/lib/python3.8/site-packages/beakerx/commands.py in
18 from .install import install, uninstall
19 from .bkr2ipynb import main
---> 20 from beakerx_magics import Py4JServer
21
22

/usr/local/lib/python3.8/site-packages/beakerx_magics/__init__.py in
22 from .sql_magic import *
23 from .jvm_kernel_magic import *
---> 24 from .sparkex_magic import *

/usr/local/lib/python3.8/site-packages/beakerx_magics/sparkex_magic.py in
19 from IPython.core.display import display
20 from IPython.core.magic import (Magics, magics_class, cell_magic)
---> 21 from beakerx.spark import (IpythonManager, BeakerxSparkServerFactory)
22 from beakerx.spark.profile import Profile
23 from beakerx.spark.spark_engine import SparkEngine

/usr/local/lib/python3.8/site-packages/beakerx/spark/__init__.py in
13 # limitations under the License.
14
---> 15 from .spark import *

/usr/local/lib/python3.8/site-packages/beakerx/spark/spark.py in
16
17 from beakerx_base import BeakerxBox
---> 18 from beakerx_magics.sparkex_widget.spark_server import BeakerxSparkServer
19 from traitlets import Unicode, List, Bool, Dict
20

/usr/local/lib/python3.8/site-packages/beakerx_magics/sparkex_widget/__init__.py in
13 # limitations under the License.
14
---> 15 from .spark_progress_bar import *

/usr/local/lib/python3.8/site-packages/beakerx_magics/sparkex_widget/spark_progress_bar.py in
12 # See the License for the specific language governing permissions and
13 # limitations under the License.
---> 14 from beakerx.button.button_widget import RESTButton
15 from beakerx_base import BeakerxBox, BaseObject, Box, HBox
16 from traitlets import Unicode, Dict, Bool

ModuleNotFoundError: No module named 'beakerx.button'

Reproduce the reported import with Jupyter Lab 1.2.9, then trace the failure through beakerx/__init__.py, commands.py, beakerx_magics/sparkex_magic.py, and spark_progress_bar.py. Check how the missing beakerx.button dependency is expected to be provided; done means `from beakerx import *` completes without ModuleNotFoundError.

Written by the indexing model from the issue text.