Adjust `test_astype`, `test_neg` for old pandas versions

• test_astype

For pandas < 1.1.0, declaring or converting to StringDtype was in general only possible if the data was already only str or nan-like (GH31204).

In pandas 1.1.0, the problem is adjusted by https://pandas.pydata.org/pandas-docs/stable/whatsnew/v1.1.0.html#all-dtypes-can-now-be-converted-to-stringdtype.

That should be considered in `test_astype`, otherwise, current tests will fail with pandas < 1.1.0.

• test_neg

dtypes = [
  "Int8",
  "Int16",
  "Int32",
  "Int64",
]
psers = []
for dtype in dtypes:
  psers.append(pd.Series([1, 2, 3, None], dtype=dtype))
  
for pser in psers:
  print((-pser).dtype)

~ 1.0.5, object dtype
1.1.0~1.1.2, TypeError: bad operand type for unary -: 'IntegerArray'
1.1.3, correct respective dtype