DataFrameGroupBy object cannot be passed to dict ctor
#### Code Sample, a copy-pastable example if possible

```python
import pandas as pd

iris = pd.read_csv("https://raw.githubusercontent.com/pandas-dev/pandas/master/pandas/tests/data/iris.csv")

groupby_obj = iris.groupby('Name')

d = dict(groupby_obj)
```

#### Problem description

Instance of `DataFrameGroupBy` is an iterable yielding pairs (*key*, *frame*),
hence it ought to work with buildin `dict`.

However, it raises error:

```
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: attribute of type 'str' is not callable
```

#### Expected Output

`d` should be a dict with groups:

```
{'Iris-setosa':     SepalLength  SepalWidth  PetalLength  PetalWidth         Name
0           5.1         3.5          1.4         0.2  Iris-setosa
1           4.9         3.0          1.4         0.2  Iris-setosa
2           4.7         3.2          1.3         0.2  Iris-setosa
3           4.6         3.1          1.5         0.2  Iris-setosa
...
7          4.6         3.2          1.4         0.2  Iris-setosa
48          5.3         3.7          1.5         0.2  Iris-setosa
49          5.0         3.3          1.4         0.2  Iris-setosa, 'Iris-versicolor':     SepalLength  SepalWidth  PetalLength  PetalWidth             Name
50          7.0         3.2          4.7         1.4  Iris-versicolor
51          6.4         3.2          4.5         1.5  Iris-versicolor
...
146          6.3         2.5          5.0         1.9  Iris-virginica
147          6.5         3.0          5.2         2.0  Iris-virginica
148          6.2         3.4          5.4         2.3  Iris-virginica
149          5.9         3.0          5.1         1.8  Iris-virginica}

```

Notice, it is possible to *workaround* this by use of intermediate
iterable:

```python
d = dict(list(groupby_obj))
```

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

<details>

```
>>> pd.show_versions()

INSTALLED VERSIONS
------------------
commit           : None
python           : 3.6.9.final.0
python-bits      : 64
OS               : Linux
OS-release       : 5.0.0-37-generic
machine          : x86_64
processor        : x86_64
byteorder        : little
LC_ALL           : None
LANG             : en_US.UTF-8
LOCALE           : en_US.UTF-8

pandas           : 0.25.3
numpy            : 1.17.4
pytz             : 2019.3
dateutil         : 2.8.1
pip              : 19.3.1
setuptools       : 42.0.2.post20191203
Cython           : None
pytest           : None
hypothesis       : None
sphinx           : None
blosc            : None
feather          : None
xlsxwriter       : None
lxml.etree       : None
html5lib         : None
pymysql          : None
psycopg2         : None
jinja2           : None
IPython          : None
pandas_datareader: None
bs4              : None
bottleneck       : None
fastparquet      : None
gcsfs            : None
lxml.etree       : None
matplotlib       : None
numexpr          : None
odfpy            : None
openpyxl         : None
pandas_gbq       : None
pyarrow          : None
pytables         : None
s3fs             : None
scipy            : None
sqlalchemy       : None
tables           : None
xarray           : None
xlrd             : None
xlwt             : None
xlsxwriter       : None

```

</details>
