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]

從 netrc.netrc 的初始化入口開始，將其目前的參數處理與提供的 io.StringIO 範例進行比較。確認應如何消費一個已開啟的 io.TextIOBase 物件，同時不變更現有輸入，然後驗證解析與 authenticator 查找仍會產生預期結果。