netrc: accept open file object as initialization argument

Feature or enhancement

Make netrc class accept open file object (io.TextIOBase subclass) as an initialization argument.

Pitch

Providing netrc data via StringIO makes it possible to retrieve it from a verity of different sources e.g. an encrypted file. With few additional lines lines of code .netrc becomes a secure password store.

import gpg

with open(file) as file_gpg:
    with gpg.Context() as c:
        plaintext = c.decrypt(file_gpg)[0]
        nrc_data = io.StringIO(plaintext.decode('utf-8'))
        nrc = netrc.netrc(nrc_data)
passwd = nrc.authenticators(hostname)[2]

从 netrc.netrc 的初始化入口开始，将其当前的参数处理与提供的 io.StringIO 示例进行比较。确认应如何消费一个已打开的 io.TextIOBase 对象，同时不改变现有输入，然后验证解析和 authenticator 查找仍会产生预期结果。