how can i calculate total working hours in django?

You can .annotate(…) [Django-doc] each Timesheet object with:

from django.db.models import DurationField, ExpressionWrapper, F, IntegerField

Timesheet.objects.annotate(
    total_time=ExpressionWrapper(
        ExpressionWrapper(F('out') - F('entry'), output_field=IntegerField()) -
        ExpressionWrapper(F('lunch_end') - F('lunch'), output_field=IntegerField()),
       output_field=DurationField()
    )
)

Here each Timesheet object that arises from this queryset will have an extra attribute .total_time which will contain the duration the person worked (so minus the luch break).

Or you can sum all records up with an .aggregate(…) [Django-doc]:

from django.db.models import DurationField, ExpressionWrapper, F, IntegerField

Timesheet.objects.aggregate(
    total_time=Sum(
        ExpressionWrapper(F('out') - F('entry'), output_field=IntegerField()) -
        ExpressionWrapper(F('lunch_end') - F('lunch'), output_field=IntegerField()),
       output_field=DurationField()
    )
)['total_time']

if the lunch and lunch_end can be None/NULL, you can work with Coalesce [Django-doc]:

from django.db.models import DurationField, ExpressionWrapper, F, IntegerField, Value
from django.db.models.functions import Coalesce

Timesheet.objects.aggregate(
    total_time=Sum(
        ExpressionWrapper(F('out') - F('entry'), output_field=IntegerField()) -
        Coalesce(ExpressionWrapper(F('lunch_end') - F('lunch'), output_field=IntegerField()), Value(0)),
       output_field=DurationField()
    )
)['total_time']

If you thus have .annotate(…)d all Timesheet records, and passed these as c, you can use:

{% for ci in c %}
    {{ ci.data }}: {{ ci.entry }} - {{ ci.out }}; {{ ci.total_time }}
{% endfor %}