depreciate ureg.__getitem__

[andrewgsavage]

I hadn't noticed a depreciate warning for ureg.getitem when using pint. This PR changes the depreciation from a logger message to DeprecationWarning.

metioned in #1303

[codspeed-hq]

CodSpeed Performance Report

Merging #1997 will improve performances by 52.29%

_{Comparing andrewgsavage:getitem (09acae1) with master (9972e60)}

Summary

⚡ 15 improvements
✅ 422 untouched benchmarks

Benchmarks breakdown

	Benchmark	master	andrewgsavage:getitem	Change
⚡	test_getitem[False-angstrom]	393.3 µs	264.7 µs	+48.57%
⚡	test_getitem[False-kilometer]	404.1 µs	274.9 µs	+47.01%
⚡	test_getitem[False-meter]	386.8 µs	259.5 µs	+49.06%
⚡	test_getitem[False-millisecond]	406.7 µs	275.7 µs	+47.5%
⚡	test_getitem[False-minute]	389.3 µs	261.8 µs	+48.69%
⚡	test_getitem[False-ms]	617 µs	497.6 µs	+24%
⚡	test_getitem[False-second]	397.2 µs	267.1 µs	+48.71%
⚡	test_getitem[True-angstrom]	395.4 µs	265.9 µs	+48.7%
⚡	test_getitem[True-kilometer]	404 µs	273 µs	+47.99%
⚡	test_getitem[True-meter]	394 µs	258.7 µs	+52.29%
⚡	test_getitem[True-millisecond]	407 µs	276.8 µs	+47.03%
⚡	test_getitem[True-minute]	396.2 µs	264.3 µs	+49.94%
⚡	test_getitem[True-ms]	623 µs	497.5 µs	+25.24%
⚡	test_getitem[True-second]	392.1 µs	259.5 µs	+51.11%
⚡	test_finding_meter_getitem	382.3 µs	256.5 µs	+49.06%

[hgrecco]

I will leave this open until just before the next minor release