title: Removing specific Position from dictionary
body: <p>I need to remove Pitchers('P') from the dictionary of potential players.
  I have tried setting </p>
  
  <p><code>team: [player for player in players_dict.keys() if player.team == team and player.positions != 'P']</code></p>
  
  <p>but it still creates a dictionary with the players in it.</p>
  
  <pre><code>class TeamStacksRule(OptimizerRule):
      def apply(self, solver, players_dict):
          stacks = self.optimizer.team_stacks
          players_by_teams = {
              team: [player for player in players_dict.keys() if player.team == team]
              for team in self.optimizer.available_teams
          }
  
          stacks_dict = {}
          for i, stack in enumerate(sorted(stacks, reverse=True), start=1):
              stacks_dict[stack] = i
  
          for stack, total in stacks_dict.items():
              combinations_variables = []
  
              for team, players in players_by_teams.items():
                  solver_variable = solver.add_variable('teams_stack_%d_%s' % (stack, team))
                  combinations_variables.append(solver_variable)
                  variables = [players_dict[player] for player in players]
                  solver.add_constraint(variables, None, SolverSign.GTE,
                                        stack * solver_variable)
              solver.add_constraint(combinations_variables, None, SolverSign.GTE, total)
  </code></pre>