Pandas Add abbrs for ['Y', 'M', 'W'] in Timedelta

Code Sample, a copy-pastable example if possible

>>> pd.Timedelta(1, 'd')
Timedelta('1 days 00:00:00')
>>> pd.Timedelta('1d')
Timedelta('1 days 00:00:00')
>>> pd.Timedelta(1, 'Y')
Timedelta('365 days 05:49:12')
>>> pd.Timedelta('1Y')
Traceback (most recent call last):
  File "pandas/_libs/tslibs/timedeltas.pyx", line 294, in pandas._libs.tslibs.timedeltas.timedelta_from_spec
KeyError: 'y'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "pandas/_libs/tslib.pyx", line 2588, in pandas._libs.tslib.Timedelta.__new__
  File "pandas/_libs/tslibs/timedeltas.pyx", line 251, in pandas._libs.tslibs.timedeltas.parse_timedelta_string
  File "pandas/_libs/tslibs/timedeltas.pyx", line 296, in pandas._libs.tslibs.timedeltas.timedelta_from_spec
ValueError: invalid abbreviation: Y

Problem description

Units beyond day (e.g., week month year) are not supported in timedelta_abbrevs, but explicitly specifying by param unit= is supported.

Is it OK to add supported of they in timedelta_abbrevs too?

Expected Output

Output of `pd.show_versions()`

[paste the output of ``pd.show_versions()`` here below this line] INSTALLED VERSIONS ------------------ commit: None python: 3.6.4.final.0 python-bits: 64 OS: Darwin OS-release: 17.3.0 machine: x86_64 processor: i386 byteorder: little LC_ALL: None LANG: zh_CN.UTF-8 LOCALE: zh_CN.UTF-8 pandas: 0.21.1 pytest: None pip: 9.0.1 setuptools: 36.5.0 Cython: None numpy: 1.13.3 scipy: 1.0.0 pyarrow: None xarray: None IPython: None sphinx: None patsy: None dateutil: 2.6.1 pytz: 2017.3 blosc: None bottleneck: None tables: None numexpr: None feather: None matplotlib: 2.1.1 openpyxl: None xlrd: None xlwt: None xlsxwriter: 1.0.2 lxml: None bs4: 4.6.0 html5lib: None sqlalchemy: None pymysql: 0.7.11.None psycopg2: None jinja2: None s3fs: None fastparquet: None pandas_gbq: None pandas_datareader: None

Comment From: jreback

see #16344 we want to remove these entirely; these are not fixed lengths of time and thus very confusing for a Timedelta.