bpo-34132: Fix netrc parsing regression

[bbayles]

This PR adds tests and a fix for the issue described in bpo-34132, a strange regression in netrc parsing.

Current versions of Python 3 to fail to parse files like this one:

# Comment
default login user password pass 

However, they can parse files like this one:

#Comment
default login user password pass 

Python 2 had no such problem - it used negative seeks instead of the lexer's line number counter. Negative seeks don't work in Python 3, but we can use absolute seeks almost as easily.
https://github.com/python/cpython/blob/2.7/Lib/netrc.py#L47-L51

I made another attempt this earlier, but this time I think I managed to avoid introducing a different regression.

https://bugs.python.org/issue34132

[serhiy-storchaka]

The result of tell() for text file is not an offset in characters. It is an opaque value, and in general case you can't do arithmetic operations with it.

Rewriting the parsing code in bpo-28806 can help to fix this issue.