Consider the following code:

$ cat bug.py
import sys
if len(sys.argv) > 1:
    sys.modules['_datetime'] = None
from datetime import tzinfo, datetime, timezone

class TZ(tzinfo):
    def utcoffset(self, t):
        pass

print(datetime(2000,1,1,tzinfo=TZ()).astimezone(timezone.utc))

When running with no arguments (with C acceleration), I get

$ ./python.exe bug.py
2000-01-01 00:00:00+00:00

but the pure python code produces an error

$ ./python.exe bug.py pure
Traceback (most recent call last):
  File "bug.py", line 10, in <module>
    print(datetime(2000,1,1,tzinfo=TZ()).astimezone(timezone.utc))
  File ".../Lib/datetime.py", line 1783, in astimezone
    raise ValueError("astimezone() requires an aware datetime")
ValueError: astimezone() requires an aware datetime

Note that some kind of error is expected because TZ.utcoffset() returns None instead of a timedelta, but the error message produced by pure python code is confusing.