fix some dotenv parsing

[dhalbert]

Fixes #6661.

This improves dotenv parsing, though there are probably still mistakes.

• # in values immediately following = are included, instead of starting a comment. This is how such values are parsed in Unix shells.
• #s inside single quote values now work properly.
• key= with a blank value are now the empty string instead of None.
• Character reading in dotenv was refactored a bit, and it's now 176 bytes smaller.
• A manual test was included.

The syntax used for dotenv files is not well specified, and there are variations in how Python dotenv works compared with how shells work, mostly revolving around comment handling.

[jepler]

Thank you! I also ran the test and it seems OK. For backslashes inside single quotes it matches dotenv, but is different from (ba)sh, but that's OK. There's a way to enter a string containing literal ' and '#' characters now and that's great.

[jepler]

thanks!