Revert use of ABC in FlightPlan.

Mypy can't handle ABC dataclasses:
https://github.com/python/mypy/issues/5374

gen/flights/flightplan.py

@@ -10,7 +10,6 @@ from __future__ import annotations
 import logging
 import math
 import random
-from abc import ABC, abstractmethod
 from dataclasses import dataclass, field
 from datetime import timedelta
 from functools import cached_property
@@ -76,7 +75,7 @@ class InvalidObjectiveLocation(PlanningError):
 
 
 @dataclass(frozen=True)
-class FlightPlan(ABC):
+class FlightPlan:
     package: Package
     flight: Flight
 
@@ -85,10 +84,9 @@ class FlightPlan(ABC):
         """A list of all waypoints in the flight plan, in order."""
         return list(self.iter_waypoints())
 
-    @abstractmethod
     def iter_waypoints(self) -> Iterator[FlightWaypoint]:
         """Iterates over all waypoints in the flight plan, in order."""
-        ...
+        raise NotImplementedError
 
     def edges(
         self, until: Optional[FlightWaypoint] = None
@@ -153,7 +151,6 @@ class FlightPlan(ABC):
         return self.flight.unit_type.fuel_consumption.cruise
 
     @property
-    @abstractmethod
     def tot_waypoint(self) -> Optional[FlightWaypoint]:
         """The waypoint that is associated with the package TOT, or None.
 
@@ -161,7 +158,7 @@ class FlightPlan(ABC):
         user-planned missions without any useful waypoints and flight plans that
         failed to generate. Nevertheless, we have to defend against it.
         """
-        ...
+        raise NotImplementedError
 
     @property
     def tot(self) -> timedelta:
@@ -239,13 +236,11 @@ class FlightPlan(ABC):
             a.position, b.position, self.speed_between_waypoints(a, b)
         )
 
-    @abstractmethod
     def tot_for_waypoint(self, waypoint: FlightWaypoint) -> Optional[timedelta]:
-        ...
+        raise NotImplementedError
 
-    @abstractmethod
     def depart_time_for_waypoint(self, waypoint: FlightWaypoint) -> Optional[timedelta]:
-        ...
+        raise NotImplementedError
 
     def request_escort_at(self) -> Optional[FlightWaypoint]:
         return None
@@ -321,10 +316,9 @@ class FlightPlan(ABC):
             return timedelta(minutes=8)
 
     @property
-    @abstractmethod
     def mission_departure_time(self) -> timedelta:
         """The time that the mission is complete and the flight RTBs."""
-        ...
+        raise NotImplementedError
 
 
 @dataclass(frozen=True)
@@ -332,23 +326,19 @@ class LoiterFlightPlan(FlightPlan):
     hold: FlightWaypoint
     hold_duration: timedelta
 
-    @abstractmethod
     def iter_waypoints(self) -> Iterator[FlightWaypoint]:
-        ...
+        raise NotImplementedError
 
     @property
-    @abstractmethod
     def tot_waypoint(self) -> Optional[FlightWaypoint]:
-        ...
+        raise NotImplementedError
 
-    @abstractmethod
     def tot_for_waypoint(self, waypoint: FlightWaypoint) -> Optional[timedelta]:
-        ...
+        raise NotImplementedError
 
     @property
-    @abstractmethod
     def push_time(self) -> timedelta:
-        ...
+        raise NotImplementedError
 
     def depart_time_for_waypoint(self, waypoint: FlightWaypoint) -> Optional[timedelta]:
         if waypoint == self.hold:
@@ -364,9 +354,8 @@ class LoiterFlightPlan(FlightPlan):
         return travel_time + self.hold_duration
 
     @property
-    @abstractmethod
     def mission_departure_time(self) -> timedelta:
-        ...
+        raise NotImplementedError
 
 
 @dataclass(frozen=True)
@@ -374,23 +363,20 @@ class FormationFlightPlan(LoiterFlightPlan):
     join: FlightWaypoint
     split: FlightWaypoint
 
-    @abstractmethod
     def iter_waypoints(self) -> Iterator[FlightWaypoint]:
-        ...
+        raise NotImplementedError
 
     @property
-    @abstractmethod
     def package_speed_waypoints(self) -> set[FlightWaypoint]:
-        ...
+        raise NotImplementedError
 
     @property
     def combat_speed_waypoints(self) -> set[FlightWaypoint]:
         return self.package_speed_waypoints
 
     @property
-    @abstractmethod
     def tot_waypoint(self) -> Optional[FlightWaypoint]:
-        ...
+        raise NotImplementedError
 
     def request_escort_at(self) -> Optional[FlightWaypoint]:
         return self.join
@@ -429,14 +415,12 @@ class FormationFlightPlan(LoiterFlightPlan):
         return self._travel_time_to_waypoint(self.join)
 
     @property
-    @abstractmethod
     def join_time(self) -> timedelta:
-        ...
+        raise NotImplementedError
 
     @property
-    @abstractmethod
     def split_time(self) -> timedelta:
-        ...
+        raise NotImplementedError
 
     def tot_for_waypoint(self, waypoint: FlightWaypoint) -> Optional[timedelta]:
         if waypoint == self.join:
@@ -498,9 +482,8 @@ class PatrollingFlightPlan(FlightPlan):
             return self.patrol_end_time
         return None
 
-    @abstractmethod
     def iter_waypoints(self) -> Iterator[FlightWaypoint]:
-        ...
+        raise NotImplementedError
 
     @property
     def package_speed_waypoints(self) -> Set[FlightWaypoint]: