Run on platforms without ssl

[Neradoc]

Catch the errors when importing ssl, and raise an exception in Server init when https is set.
This will allow using httpserver on ESP32SPI with adafruit/Adafruit_CircuitPython_ESP32SPI#218
Fixes #102

[anecdata]

The hashlib import prints the warning right away, as @michalpokusa points out in the issue:
https://github.com/adafruit/Adafruit_CircuitPython_HTTPServer/blob/09e5431071d9e484726df87841a5b9bbe33b6d76/adafruit_httpserver/response.py#L20-29

Should these both be done the same way (whichever of the two), or is there a reason to do them differently?

[Neradoc]

The warning for hashlib can be overcome by installing the library in question. Installing ssl is not an option (other than creating a dummy ssl.py for example), so this would only be a forced print every time with no purpose.

In fact I would argue that the warning for hashlib should be removed but I chose not to do it in this PR. It doesn't bring anything useful when not using websockets, and it's redundant with the error that will happen when trying to. In general such prints should not happen unless there's a "debug" or "verbose" option set somewhere.

For SSL it would be also interesting in the future to support other implementations by injecting it as a dependency and import the necessary libraries only when desired, but that's way beyond the scope of this PR.