BUG: passing a non-dict mapping to pd.concat raises a TypeError
#### Code Sample, a copy-pastable example if possible

```python
class MyMapping(Mapping):
    def __init__(self, content=None):
        if content:
            self._content = content
        else:
            self._content = {}
    def __getitem__(self, key):
        return self._content[key]
    def __iter__(self):
        return iter(self._content)
    def __len__(self):
        return len(self._content)   

objs = MyMapping({"a": pd.DataFrame({"_0": [0.], "_1": [0.]}), "b": pd.DataFrame({"_0": [0.], "_1": [0.]})}
concat_objs = pd.concat(objs, axis=1)
```
#### Problem description

The above code raises a TypeError with the message "cannot concatenate object of type '<class 'str'>'; only Series and DataFrame objs are valid".

As the documentation explicitly specifies that the argument `objs` can be a mapping of Series or DataFrame objects, the expected behaviour is to just proceed.

#### Expected Output

The concatenated data frame.


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

<details>

INSTALLED VERSIONS
------------------
commit           : None
python           : 3.6.7.final.0
python-bits      : 64
OS               : Darwin
OS-release       : 19.3.0
machine          : x86_64
processor        : i386
byteorder        : little
LC_ALL           : None
LANG             : None
LOCALE           : en_GB.UTF-8
pandas           : 0.25.3
numpy            : 1.18.1
pytz             : 2019.3
dateutil         : 2.8.1
pip              : 19.3.1
setuptools       : 41.0.0
Cython           : None
pytest           : 5.3.2
hypothesis       : None
sphinx           : 1.8.5
blosc            : None
feather          : None
xlsxwriter       : None
lxml.etree       : 4.3.1
html5lib         : None
pymysql          : None
psycopg2         : None
jinja2           : 2.11.1
IPython          : 7.12.0
pandas_datareader: None
bs4              : None
bottleneck       : 1.2.1
fastparquet      : None
gcsfs            : None
lxml.etree       : 4.3.1
matplotlib       : 3.1.3
numexpr          : 2.7.1
odfpy            : None
openpyxl         : None
pandas_gbq       : None
pyarrow          : None
pytables         : None
s3fs             : None
scipy            : 1.4.1
sqlalchemy       : None
tables           : 3.6.1
xarray           : None
xlrd             : None
xlwt             : None
xlsxwriter       : None
</details>
