feat: add delete_sub_uid() & removed utcnow()

[LordGhostX]

• Add a method named delete_sub_uid which implements Delete Sub UID for deleting a subaccount with its UID
• Replace references of dt.utcnow() with dt.now(timezone.utc) following its deprecation: https://blog.miguelgrinberg.com/post/it-s-time-for-a-change-datetime-utcnow-is-now-deprecated
• Update Delete Sub UID documentation (Docusaurus Source) to include an example for the Python SDK:

from pybit.unified_trading import HTTP
session = HTTP(
    testnet=True,
    api_key="XXXXX",
    api_secret="XXXXX",
)
print(session.delete_sub_uid(
    subMemberId="XXXXX"
))

Here's a use case for Delete Sub UID. I manage a codebase that automates the following processes:

• Create subaccounts
• Configure subaccounts (hedge mode, leverage, partial SL mode, isolated mode)
• Generate their API keys
• Delete expired keys and generate new ones
• Transfer assets between subaccounts
• Place orders and track PnL on subaccounts

My tests must ensure that subaccounts can be created and configured successfully. Once this is confirmed, it deletes the subaccount.

cc: @dextertd @woodliang10