[REVIEW] Don't identify decimals as strings.

[vyasr]

As documented in this pandas issue, is_string_type for pandas is not strict and will characterize a whole bunch of things as strings that aren't. For our purposes, this is problematic because basically all subclasses of ExtensionDType will be classified as strings by that function. This is definitely not appropriate, so I modified our version of is_string_dtype to explicitly reject all of our extension dtypes (previously it was only excluding categorical types). I'm not 100% confident that no other parts of the code base rely on the current (erroneous) behavior, but the cudf tests all passed for me locally and my attempt to trace all calls of utils.is_string_dtype all look to be places where the change gives more correct behavior, so I think our best bet is to just move forward with this change. Any problems that result from this change in the future due to other code relying on the current behavior should probably be characterized as bugs in the calling code and fixed there. The same goes for for external codes that relied on this behavior; this change is potentially breaking for them as well, but again is something that they should be addressing.

[codecov]

Codecov Report

Merging #7710 (babcdfc) into branch-0.19 (7871e7a) will increase coverage by 0.64%.
The diff coverage is n/a.

@@               Coverage Diff               @@
##           branch-0.19    #7710      +/-   ##
===============================================
+ Coverage        81.86%   82.51%   +0.64%     
===============================================
  Files              101      101              
  Lines            16884    17450     +566     
===============================================
+ Hits             13822    14398     +576     
+ Misses            3062     3052      -10     

Impacted Files	Coverage Δ
python/cudf/cudf/core/buffer.py	84.21% <ø> (+4.96%)	⬆️
python/cudf/cudf/core/column/categorical.py	91.97% <ø> (+0.58%)	⬆️
python/cudf/cudf/core/column/column.py	87.61% <ø> (-0.15%)	⬇️
python/cudf/cudf/core/column/datetime.py	89.73% <ø> (+0.63%)	⬆️
python/cudf/cudf/core/column/decimal.py	92.95% <ø> (-1.92%)	⬇️
python/cudf/cudf/core/column/lists.py	90.00% <ø> (-1.40%)	⬇️
python/cudf/cudf/core/column/numerical.py	94.83% <ø> (-0.20%)	⬇️
python/cudf/cudf/core/column/string.py	86.79% <ø> (+0.30%)	⬆️
python/cudf/cudf/core/column/timedelta.py	88.66% <ø> (+0.42%)	⬆️
python/cudf/cudf/core/column_accessor.py	96.13% <ø> (+0.82%)	⬆️
... and 58 more

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[vyasr]

Looks like this is a slightly more subtle problem. There's more to be done here to actually enable decimal operations; the way that they're currently be rejected is problematic (what I fixed above), but they are currently being rejected intentionally and I'll have to do a bit more work to actually enable them.

[kkraus14]

@gpucibot merge