Add Mythic item passive bonus

[PaulQbFeng]

• Now that some Mythic items are coded
• The item type has also been introduced
• Need method to apply mythic passive during inventory init and while equipping an item

[LinSun00]

• A champion can only equip one mythic

[LinSun00]

What do you think about making inventory a class ? It can keep track of the number of items of each item_type. This way, we can easily apply the mythic passive according to the number of legendary items in the inventory. We can also define add and remove methods which will be called respectively during equip and unequip in BaseChampion. Other features could include a get_stat method. This would also help making the BaseChampion class a bit cleaner, since it is starting to have a lot of random stuff which makes it a bit confusing imo.

[LinSun00]

Another feature missing in the Inventory branch:

• There are other limitations outside of "only one mythic per champion". Other types include Lifeline (immortal shieldbow, sterak, maw), immolate (interesting to note that in this case, the unique passive doesn't only mean that the passive can't be applied twice, but that the item itself can't be equipped twice), etc.
• A champion can't have more than 1 copy of every legendary item

[LinSun00]

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