Defer import of dask.array to speed up import

[ericpre]

When various array backends of dask are installed, it can slow down significantly the import time of pint.

Running python -X importtime -c "from pint import UnitRegistry" 2> pint.txt && tuna pint.txt

Without this PR:

With this PR:

• [n/a] Closes # (insert issue number)
• Executed pre-commit run --all-files with no errors
• The change is fully covered by automated unit tests
• [n/a] Documented in docs/ as appropriate
• Added an entry to the CHANGES file

[hgrecco]

awesome improvement. I have a few suggestions that I will add as comments in the code

[ericpre]

@hgrecco, when you have a chance, can you please add your comments to this PR? Thanks!

[ericpre]

@hgrecco, would it possible to get this PR reviewed?

[andrewgsavage]

closed in favour of #2276