Repeated capability accessors return internal `_SessionBase` type

Description of issue

Repeated capability accessors like Session.channels and Session.instruments return a _SessionBase object. Clients that pass channel objects around may need to refer to this type in PEP 484 type hints.

Example: https://github.com/ni/measurementlink-python/blob/9bffe48c1c2da4777eeca0ca159b74bde724ead6/examples/output_voltage_measurement/measurement.py#L177

def _wait_for_source_complete_event(
    measurement_service: nims.MeasurementService,
    channels: nidcpower._SessionBase,
    cancellation_event: threading.Event,
) -> None: ...

The convention recommended in PEP8 and used by most Python code is that names with a single leading underscore are for internal use.

In general, public APIs ought to use public types.

System report

Not applicable.

Steps to reproduce issue

1. Write a function that accepts a channel object (such as the linked example).
2. Write PEP 484 type hints for this function.

Start by locating the implementations and type annotations for repeated capability accessors such as Session.channels and Session.instruments, then compare them with the usage in examples/output_voltage_measurement/measurement.py around line 177. Done means clients can use a public type for channel objects in PEP 484 annotations instead of referring to the internal _SessionBase type.