Where has the "units" sub-module in qiskit_nature gone?

I am trying to follow this qiskit "Electronic Structure" tutorial, but at the very first step, setting up the PySCF driver with the following

from qiskit_nature.units import DistanceUnit
from qiskit_nature.second_q.drivers import PySCFDriver

driver = PySCFDriver(
    atom="H 0 0 0; H 0 0 0.735",
    basis="sto3g",
    charge=0,
    spin=0,
    unit=DistanceUnit.ANGSTROM,
)

produces an error. The error says there is no module "units" in qiskit_nature.

I have installed qiskit_nature with pip (pip3, python 3.6) and when I listed the submodules of qiskit_nature I get

algorithms
circuit
constants
converters
deprecation
drivers
exceptions
mappers
operators
problems
properties
results
runtime
settings
transformers
version

I checked most and there appears not to be a "units" module anywhere. While the call to the submodule "units" in the above may be trivial and/or circumvented, where has the submodule gone? Is it missing because when they ported the "qiskit.chemistry" to "nature" they re-organised things? If so, why do the tutorials on the "nature" module not reflect this?

The module units.py is there in Qiskit Nature in the root. There is no package/folder/submodule called units. Its a single file.

pip install --upgrade qiskit_nature

maybe work around it, replace the import statement from qiskit_nature.units import DistanceUnit with qiskit_nature.units._units

I am assuming that you are trying to run the latest tutorial but you do not have the latest version of Qiskit Nature installed. This is based on three observatons:

1. The units module is missing which only got added in Qiskit Nature v0.5.0
2. The second_q module is missing, too, which also got added in that version.
3. Furthermore, you say that you are using Python 3.6, support which has been removed in version 0.4.0.

Thus, I strongly assume you installed Qiskit Nature v0.3.2 which is the last release that pip should resolve to under that Python version.

Now, how do you go about solving this problem: I strongly suggest that you first switch to a newer Python version. 3.6 has reached its end of life on Dec 23rd, 2021 1.
I suggest you upgrade to Python 3.9 which will be supported into 2025 and has already been "battle-tested" on all platforms.

Once you have upgrade your Python version, you will need to re-install Qiskit Nature:

pip3 install --upgrade qiskit-nature