BUG: DatetimeIndex de-serializing fails in PYTHONOPTIMIZE mode (python -O)
- [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.

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

---


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

Run python as PYTHONOPTIMIZE, option -O:

```bash
python -O
```

```python
import pickle
import pandas as pd

idx = pd.date_range('2021-01-01', '2021-01-02', freq='D')
idx_dump = pickle.dumps(idx)
pickle.loads(idx_dump)

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File ".../lib/python3.7/site-packages/pandas/core/indexes/datetimes.py", line 61, in _new_DatetimeIndex
    result = cls._simple_new(dta, **d)
TypeError: _simple_new() got an unexpected keyword argument 'tz'

```

#### Problem description

These dict operations go under the 'assert' statement:

```python

# pandas/core/indexes/datetimes.py

def _new_DatetimeIndex(cls, d):
    ...
        if key in d:
            assert d.pop(key) == getattr(dta, key)

    ...

```

which are ignored in PYTHONOPTIMIZED mode.
As a fix, bring dict pop operation out of the assert statement.

#### Expected Output

```python

DatetimeIndex(['2021-01-01', '2021-01-02'], dtype='datetime64[ns]', freq='D')

```

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

<details>

INSTALLED VERSIONS
------------------
commit           : 7c48ff4409c622c582c56a5702373f726de08e96
python           : 3.7.9.final.0
python-bits      : 64
OS               : Darwin
OS-release       : 20.5.0
Version          : Darwin Kernel Version 20.5.0: Sat May  8 05:10:33 PDT 2021; root:xnu-7195.121.3~9/RELEASE_X86_64
machine          : x86_64
processor        : i386
byteorder        : little
LC_ALL           : None
LANG             : en_GB.UTF-8
LOCALE           : en_GB.UTF-8

pandas           : 1.2.5
numpy            : 1.20.3
pytz             : 2021.1
dateutil         : 2.8.1
pip              : 21.1.2
setuptools       : 57.0.0
Cython           : None
pytest           : None
hypothesis       : None
sphinx           : None
blosc            : None
feather          : None
xlsxwriter       : None
lxml.etree       : 4.6.3
html5lib         : 1.1
pymysql          : None
psycopg2         : 2.8.6 (dt dec pq3 ext lo64)
jinja2           : None
IPython          : 7.20.0
pandas_datareader: None
bs4              : None
bottleneck       : None
fsspec           : None
fastparquet      : None
gcsfs            : None
matplotlib       : None
numexpr          : None
odfpy            : None
openpyxl         : 3.0.7
pandas_gbq       : None
pyarrow          : None
pyxlsb           : None
s3fs             : None
scipy            : 1.6.3
sqlalchemy       : None
tables           : None
tabulate         : None
xarray           : None
xlrd             : 1.2.0
xlwt             : None
numba            : None

</details>
