Add static type checking via Mypy

[shwina]

Adds static type checking to cuDF Python via MyPy.

• An additional mypy style check is enabled in CI
• mypy is run as part of the pre-commit hook
• Many parts of the cuDF internal code now have type annotations
• Any new internal code is expected to be written with type annotations (not public-facing APIs)

[GPUtester]

Please update the changelog in order to start CI tests.

View the gpuCI docs here.

[codecov]

Codecov Report

Merging #6381 (06aef48) into branch-0.18 (8860baf) will increase coverage by 0.08%.
The diff coverage is 100.00%.

@@               Coverage Diff               @@
##           branch-0.18    #6381      +/-   ##
===============================================
+ Coverage        82.09%   82.17%   +0.08%     
===============================================
  Files               97       99       +2     
  Lines            16474    16805     +331     
===============================================
+ Hits             13524    13810     +286     
- Misses            2950     2995      +45     

Impacted Files	Coverage Δ
python/cudf/cudf/_lib/__init__.py	100.00% <ø> (ø)
python/cudf/cudf/_typing.py	91.66% <ø> (ø)
python/cudf/cudf/core/__init__.py	100.00% <ø> (ø)
python/cudf/cudf/core/abc.py	91.48% <ø> (+4.25%)	⬆️
python/cudf/cudf/core/buffer.py	80.00% <ø> (+0.95%)	⬆️
python/cudf/cudf/core/column/__init__.py	100.00% <ø> (ø)
python/cudf/cudf/core/column/categorical.py	92.73% <ø> (-0.62%)	⬇️
python/cudf/cudf/core/column/column.py	87.75% <ø> (-0.39%)	⬇️
python/cudf/cudf/core/column/datetime.py	89.62% <ø> (+0.19%)	⬆️
python/cudf/cudf/core/column/decimal.py	94.87% <ø> (ø)
... and 68 more

---

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[shwina]

rerun tests

[kkraus14]

@gpucibot merge

[galipremsagar]

rerun tests

[shwina]

Rerun tests

[kkraus14]

Looks like a flaky test related to the groupby sorting change:

def test_serialize_groupby_external():
        df = cudf.DataFrame()
        df["val"] = np.arange(100, dtype=np.float32)
        gb = df.groupby(cudf.Series(np.random.randint(0, 20, 100)))
        outgb = gb.deserialize(*gb.serialize())
        expect = gb.mean()
        got = outgb.mean()
>       assert_eq(got, expect)

cudf/tests/test_serialize.py:163: 
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 
cudf/tests/utils.py:87: in assert_eq
    tm.assert_frame_equal(left, right, **kwargs)
pandas/_libs/testing.pyx:67: in pandas._libs.testing.assert_almost_equal
    ???
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 

>   ???
E   AssertionError: DataFrame.index are different
E   
E   DataFrame.index values are different (10.0 %)
E   [left]:  Int64Index([5, 17, 13, 15, 16, 18, 12, 6, 1, 7, 8, 10, 0, 19, 3, 14, 2, 11, 4,
E               9],
E              dtype='int64')
E   [right]: Int64Index([5, 17, 13, 15, 16, 18, 12, 6, 1, 7, 8, 0, 10, 19, 3, 14, 2, 11, 4,
E               9],
E              dtype='int64')

pandas/_libs/testing.pyx:182: AssertionError

[shwina]

Looking into it

[cwharris]

Partial review. This is great. :)

[kkraus14]

@gpucibot merge

[cwharris]

Is there any reason for tests to provide type annotations? (annotations for test utilities, I can see being helpful, but not limited-scope test code).