Don't close asyncio subprocess stream if it's stdin is closed

[asvetlov]

BPO	36805
Nosy	@asvetlov, @1st1

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields: ```python assignee = None closed_at = None created_at = labels = ['type-bug', '3.8', 'expert-asyncio'] title = "Don't close subprocess stream if it's stdin is closed" updated_at = user = 'https://github.com/asvetlov' ``` bugs.python.org fields: ```python activity = actor = 'asvetlov' assignee = 'none' closed = False closed_date = None closer = None components = ['asyncio'] creation = creator = 'asvetlov' dependencies = [] files = [] hgrepos = [] issue_num = 36805 keywords = [] message_count = 1.0 messages = ['341467'] nosy_count = 2.0 nosy_names = ['asvetlov', 'yselivanov'] pr_nums = [] priority = 'normal' resolution = None stage = None status = 'open' superseder = None type = 'behavior' url = 'https://bugs.python.org/issue36805' versions = ['Python 3.8'] ```

[asvetlov]

Closing stdin FD by child stream is a legal operation, there is no reason to close entire subprocess transport immediately.