Add dotenv read support

[tannewt]

os.getenv() will use it (when available) to load variables from
/.env

This will also be useful when we need secrets or config for
CircuitPython outside of the VM (like WiFi credentials.)

Fixes #4212

[tannewt]

Tested with .env:

key=value
  key2 = value2
'key3' = 'value with spaces  '
# comment
key4 = value3 # comment 2
'key5'=value4
key=value5 # overrides the first one
key 6 = value 6 # won't be found
key7 = value unquoted # will go to end?
multiline = 'hello
world
how are you?'
key8 = ' escaped \\ \' '

And code.py:

import time

# time.sleep(10)

import dotenv

keys = ["wifi_ssid", "wifi_password", "foo", "key", "key1", "key2", "key3", "key4", "key5", "key 6", "key7", "key8", "multiline"]

for key in keys:
    print(key, repr(dotenv.get_key(".env", key)))

[dhalbert]

I tested plain values, single-quoted values, and multi-line values, and they work for me.

But it doesn't always take the last value for multiple values:

abc=def
abc=ghi

and it gives me the first value, not the second

Adafruit CircuitPython 8.0.0-alpha.0-6-g3dca8a776 on 2022-05-23; Adafruit Metro M4 Express with samd51j19
>>> import dotenv
>>> dotenv.get_key(".env", "abc")
'def'

However, this gives the last value:

abc=def
abc=ghi
abc=jkl

>>> dotenv.get_key(".env", "abc")
'jkl'

abc=def
abc=ghi
abc=jkl
abc=mno

>>> dotenv.get_key(".env", "abc")
'jkl'

So some odd/even thing?

[dhalbert]

Re-tested with fix. Works great. Also works with a file without a final newline, and with \r\n linebreaks. Thanks - this is going to be be very useful!