BUG: rolling count on string Series and TimeIndex raises an error
- [x] I have checked that this issue has not already been reported.

- [x] I have confirmed this bug exists on the latest version of pandas.

- [ ] (optional) I have confirmed this bug exists on the master branch of pandas.

#### Code Sample, a copy-pastable example

```python
d = {'vv': {pd.Timestamp('2020-01-13 08:22:00', freq='T'): 'aa',
  pd.Timestamp('2020-01-13 08:23:00', freq='T'): 'bb',
  pd.Timestamp('2020-01-13 08:24:00', freq='T'): 'cc',
  pd.Timestamp('2020-01-13 08:25:00', freq='T'): np.nan,
  pd.Timestamp('2020-01-13 08:26:00', freq='T'): 'dd'}}

df = pd.DataFrame(d)

print(df['vv'].rolling('72s').count())

```

#### Problem description

this raises DataError: No numeric types to aggregate insted if the expected output:

    2020-01-13 08:22:00    1.0
    2020-01-13 08:23:00    2.0
    2020-01-13 08:24:00    2.0
    2020-01-13 08:25:00    1.0
    2020-01-13 08:26:00    1.0
    Name: vv, dtype: float64

#### Output of ``pd.show_versions()``

<details>

INSTALLED VERSIONS
------------------
commit           : d9fff2792bf16178d4e450fe7384244e50635733
python           : 3.8.5.final.0
python-bits      : 64
OS               : Windows
OS-release       : 10
Version          : 10.0.16299
machine          : AMD64
processor        : Intel64 Family 6 Model 78 Stepping 3, GenuineIntel
byteorder        : little
LC_ALL           : None
LANG             : None
LOCALE           : English_United States.1252

pandas           : 1.1.0
numpy            : 1.19.1
pytz             : 2020.1
dateutil         : 2.8.1
pip              : 20.2.1
setuptools       : 49.2.1.post20200807
Cython           : 0.29.21
pytest           : 6.0.1
hypothesis       : None
sphinx           : 3.2.0
blosc            : None
feather          : None
xlsxwriter       : 1.2.9
lxml.etree       : 4.5.2
html5lib         : 1.1
pymysql          : None
psycopg2         : None
jinja2           : 2.11.2
IPython          : 7.16.1
pandas_datareader: None
bs4              : 4.9.1
bottleneck       : 1.3.2
fsspec           : 0.7.4
fastparquet      : None
gcsfs            : None
matplotlib       : 3.2.2
numexpr          : 2.7.1
odfpy            : None
openpyxl         : 3.0.4
pandas_gbq       : None
pyarrow          : None
pytables         : None
pyxlsb           : None
s3fs             : None
scipy            : 1.5.0
sqlalchemy       : 1.3.18
tables           : 3.6.1
tabulate         : None
xarray           : None
xlrd             : 1.2.0
xlwt             : 1.3.0
numba            : 0.50.1

</details>
