DHCP timeout and hostname fix, make both user settable

[xorbit]

• DHCP timeouts were a bit of a mess, the code was cleaned up.
• The default was made 3 seconds vs 1 second which was too short
  for some DHCP servers.
• The DHCP timeout is now settable with a new dhcp_timeout
  parameter for the WIZNET5K class.
• DHCP hostname was set wrong, by trying to add the binary version
  of the MAC, it became corrupt and was rejected by the DHCP server.
  Default is now fixed, "WIZnetXXXXXXXXXXXX" where X'es are MAC.
• DHCP hostname is now settable on WIZNET5K class using 'hostname'
  keyword argument. If the string contains '{}', the MAC will be
  inserted in this position.

[brentru]

@xorbit Ok, this looks great and works well. Thank you for the PR!

---

Tests OK with simpletest on Adafruit CircuitPython 5.3.0 on 2020-04-29; Adafruit Feather Bluefruit Sense with nRF52840

Wiznet5k WebClient Test
Chip Version: w5500
MAC Address: ['0xde', '0xad', '0xbe', '0xef', '0xfe', '0xed']
My IP address is: 
IP lookup adafruit.com: 104.20.38.240
Fetching text from http://wifitest.adafruit.com/testwifi/index.html
----------------------------------------
This is a test of Adafruit WiFi!
If you can read this, its working :)

----------------------------------------

Fetching json from http://api.coindesk.com/v1/bpi/currentprice/USD.json
----------------------------------------
{'time': {'updated': 'Jun 5, 2020 15:32:00 UTC', 'updatedISO': '2020-06-05T15:32:00+00:00', 'updateduk': 'Jun 5, 2020 at 16:32 BST'}, 'disclaimer': 'This data was produced from the CoinDesk Bitcoin Price Index (USD). Non-USD currency data converted using hourly conversion rate from openexchangerates.org', 'bpi': {'USD': {'code': 'USD', 'description': 'United States Dollar', 'rate_float': 9666.01, 'rate': '9,666.0112'}}}
----------------------------------------
Done!

I also tested setting a custom hostname:
eth = WIZNET5K(spi_bus, cs, hostname="blinka")

The custom hostname displays in my router's DHCP server client listing:

[brentru]

This is failing on the CI's formatting check, please check out this page for instructions for using Black formatting: https://learn.adafruit.com/improve-your-code-with-pylint/black?