Allow installation of current 'structlog' major version 21.x

[grelleum]

This PR allows using structlog major version 21 to be installed with diffsync.
This fixes issue: #74

Three changes are made:

1. Pinned the pylint version to 2.6.x so that invoke tests will complete successfully after running poetry lock.
2. Modified structlog versioning to allow 20.x.x or 21.x.x. Ran complete tests to verify compatibility.
3. Updated poetry.lock file to include current structlog version.

[dgarros]

Looks like there is a breaking change that we need to account for
https://www.structlog.org/en/stable/changelog.html#backward-incompatible-changes

I did a quick search and we are using format_exc_info in

diffsync/diffsync/logging.py

Line 45 in 3c3060f

structlog.processors.format_exc_info,

[grelleum]

Thanks @dgarros - I will look further into this as see how these changes affects DiffSync.

[glennmatthews]

This looks pretty gross. Can we use packaging.Version objects here instead? Something like:

from packaging import version

if version.parse(structlog.__version__) < version.Version("21.2.0"):

[grelleum]

Great suggestion - I've implemented the change.

[glennmatthews]

Don't we need a processors=processors, line added here?

[grelleum]

Indeed, it got lost somewhere in the experimentation phase. Added this back in.

[grelleum]

The structlog change in 21.2.0 and above changes behavior with regards to the use of format_exc_info as a log processor.
This effects exception logging when using the logging.enable_console_logging function.

When using 21.2.0 or newer and either better_exceptions or rich in available in the Python environment, then exception logging will generate a warning if structlog.processors.format_exc_info is installed as a processor. If we remove the format_exc_info processor then we lose exception traceback formatting, when the above is not the case.

The code added here determines if the structlog.processors.format_exc_info is required in the processor list, and appends it to the list if required.

[grelleum]

Found another issue in my code.

    rich = importlib.util.find_spec("rich")
    better_exceptions = importlib.util.find_spec("better_exceptions")

It looks like importlib.util has been removed from Python 3.9.
I am investigating the proper way to determine if a library is installed without actually importing the library.

[grelleum]

For compatibility with Python 3.9, I changed:

import importlib
...
    rich = importlib.util.find_spec("rich")
    better_exceptions = importlib.util.find_spec("better_exceptions")

to

from importlib.util import find_spec
... 
    rich = find_spec("rich")
    better_exceptions = find_spec("better_exceptions")

[dgarros]

LGTM thank you @grelleum for the PR
@glennmatthews do you think we are good to merge ?

[glennmatthews]