Importing code generator changes behavior of pyyaml

[gluap]

When importing datamodel_code_generator, the behaviour of pyyaml is changed after the import, datetime objects suddenly are deserialized from yaml as str whereas before they were deserialized as datetime objects.

Steps to reproduce:

create clean venv

python3 -m venv blub
blub/bin/pip install datamodel-code-generator pyyaml

Run this reproducer with the interpreter from the venv

import yaml
import tempfile
import datetime

tmp=tempfile.NamedTemporaryFile(mode='w')

def write_read():
    tmp=tempfile.NamedTemporaryFile(mode='w')
    yaml.safe_dump({"datetime": datetime.datetime.now()}, tmp)
    dumped_reloaded = yaml.safe_load(open(tmp.name,"r"))
    print(dumped_reloaded)

print("before importing datamodel-code-generator:")
write_read()

import datamodel_code_generator
print("\nafter importing datamodel-code-generator:")
write_read()

Actual output: (time string + object may change depending on run time)

before importing datamodel-code-generator:
{'datetime': datetime.datetime(2024, 1, 30, 9, 51, 19, 612368)}

after importing datamodel-code-generator:
{'datetime': '2024-01-30 09:51:20.019991'}

Expected output: (time string + object may change depending on run time):

before importing datamodel-code-generator:
{'datetime': datetime.datetime(2024, 1, 30, 9, 51, 19, 612368)}

after importing datamodel-code-generator:
{'datetime': datetime.datetime(2024, 1, 30, 9, 51, 20, 019991)}

The reason seems to be that the SafeLoader of yaml is modified globally by datamodel-code-generator

datamodel-code-generator/datamodel_code_generator/util.py

Line 72 in acc6bf6

SafeLoader.yaml_constructors[