Change requirements to pypi format, remove ESP32SPI requirement

[Neradoc]

The library does not need ESP32SPI, it works on native wifi too. The format used for the minimqtt requirement makes it listed in "external_dependencies" instead of "dependencies" in the bundle's json file. It might be what is tripping the project bundler for the learn guides using adafruit_io, or indirectly via portalbase. At least that's my assumption.

adafruit_minimqtt is missing from the Project Bundle there for example:

• https://learn.adafruit.com/magtag-weather/project-code
• https://learn.adafruit.com/pyportal-weather-station/code-pyportal-with-circuitpython

[tekktrik]

Thanks!