BUG: AttributeError: 'BlockManager' object has no attribute 'is_mixed_type' when trying to jsonify dataframe.
- [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
csv_reader = pandas.read_csv(
             csv_file, iterator=True, chunksize=2500, sep=',', encoding=None, error_bad_lines=False
     )
for i, chunk in enumerate(csv_reader):
     rows = chunk.to_json(orient="records", lines=True)
```

#### Problem description
**on pypy 7.3.3 - python 3.7.9**
`to_json` execution raises an exception:
```
Traceback (most recent call last):
  File "<stdin>", line 2, in <module>
  File "/opt/pypy/site-packages/pandas/core/generic.py", line 2478, in to_json
    storage_options=storage_options,
  File "/opt/pypy/site-packages/pandas/io/json/_json.py", line 94, in to_json
    indent=indent,
  File "/opt/pypy/site-packages/pandas/io/json/_json.py", line 155, in write
    indent=self.indent,
AttributeError: 'BlockManager' object has no attribute 'is_mixed_type'
```
I suspect this PR: https://github.com/pandas-dev/pandas/pull/36873/files
@jbrockmendel do you mind explaining why the `BlockManager` no longer has the `is_mixed_type` property?

#### Expected Output
When I try to manually assign `is_mixed_type` to the `chunk._mgr` object, as dropped in the aforementioned PR (a `BlockManager`) it seems to be working fine:
```
for i, chunk in enumerate(csv_reader):
     chunk._mgr._consolidate_inplace()
     chunk._mgr.is_mixed_type = len(chunk._mgr.blocks) > 1
     print(chunk.to_json(orient="records", lines=True))

{"@timestamp":10000000,"test_1":"something","test_2":"something2"}
```


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

INSTALLED VERSIONS
------------------
commit           : 7d32926db8f7541c356066dcadabf854487738de
python           : 3.7.9.final.0
python-bits      : 64
OS               : Linux
OS-release       : 4.19.121-linuxkit
Version          : #1 SMP Tue Dec 1 17:50:32 UTC 2020
machine          : x86_64
processor        :
byteorder        : little
LC_ALL           : None
LANG             : C.UTF-8
LOCALE           : en_US.UTF-8

pandas           : 1.2.2
numpy            : 1.20.1
pytz             : 2021.1
dateutil         : 2.8.1
pip              : 20.3.4
setuptools       : 53.0.0
Cython           : None
pytest           : 6.2.2
hypothesis       : None
sphinx           : None
blosc            : None
feather          : None
xlsxwriter       : None
lxml.etree       : None
html5lib         : None
pymysql          : None
psycopg2         : 2.8.6 (dt dec pq3 ext lo64)
jinja2           : 2.11.3
IPython          : 7.20.0
pandas_datareader: None
bs4              : None
bottleneck       : None
fsspec           : None
fastparquet      : None
gcsfs            : None
matplotlib       : None
numexpr          : None
odfpy            : None
openpyxl         : None
pandas_gbq       : None
pyarrow          : None
pyxlsb           : None
s3fs             : None
scipy            : None
sqlalchemy       : 1.3.23
tables           : None
tabulate         : 0.8.7
xarray           : None
xlrd             : 1.2.0
xlwt             : None
numba            : None

</details>
