ExtensionArray._can_hold_na needs to be a class attribute

[Dr-Irv]

Problem description

ExtensionArray._can_hold_na is implemented as an instance property, but needs to be a class attribute to be consistent with its use throughout pandas.

(Note - I am working on a PR that will take care of this)

Output of pd.show_versions()

INSTALLED VERSIONS ------------------ commit: f463cf26b35ec11ae4fdf332635d6e89a9db2f17 python: 3.6.4.final.0 python-bits: 64 OS: Windows OS-release: 10 machine: AMD64 processor: Intel64 Family 6 Model 60 Stepping 3, GenuineIntel byteorder: little LC_ALL: None LANG: None LOCALE: None.None

pandas: 0.23.0.dev0+747.gf463cf26b.dirty
pytest: 3.4.0
pip: 9.0.1
setuptools: 38.5.1
Cython: 0.25.1
numpy: 1.14.1
scipy: 1.0.0
pyarrow: 0.8.0
xarray: None
IPython: 6.2.1
sphinx: 1.7.1
patsy: 0.5.0
dateutil: 2.6.1
pytz: 2018.3
blosc: 1.5.1
bottleneck: 1.2.1
tables: 3.4.2
numexpr: 2.6.4
feather: None
matplotlib: 2.2.0
openpyxl: 2.5.0
xlrd: 1.1.0
xlwt: 1.3.0
xlsxwriter: 1.0.2
lxml: 4.1.1
bs4: 4.6.0
html5lib: 1.0.1
sqlalchemy: 1.2.5
pymysql: 0.8.0
psycopg2: None
jinja2: 2.10
s3fs: 0.1.3
fastparquet: None
pandas_gbq: None
pandas_datareader: None

[TomAugspurger]

@Dr-Irv do you have an ETA for your PR?

[Dr-Irv]

@TomAugspurger Made good progress this morning, so I'm pretty sure I will submit by late afternoon today (Monday, 4/26). I'm sure that you and others will have lots of changes to request!