Use threadpool for reading from file for sendfile fallback mode

[asvetlov]

BPO	37280
Nosy	@asvetlov, @1st1
PRs	bpo-37280: Use threadpool for reading from file in sendfile fallback mode #14076 [3.8] bpo-37280: Use threadpool for reading from file in sendfile fallback mode (GH-14076) #14102 [3.7] bpo-37280: Use threadpool for reading from file in sendfile fallback mode (GH-14076) #14101

^{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:

assignee = None
closed_at = <Date 2019-06-15.11:26:34.630>
created_at = <Date 2019-06-14.10:29:49.113>
labels = ['3.8', 'type-bug', '3.7', '3.9', 'expert-asyncio']
title = 'Use threadpool for reading from file for sendfile fallback mode'
updated_at = <Date 2019-06-15.11:26:34.630>
user = 'https://github.com/asvetlov'

bugs.python.org fields:

activity = <Date 2019-06-15.11:26:34.630>
actor = 'asvetlov'
assignee = 'none'
closed = True
closed_date = <Date 2019-06-15.11:26:34.630>
closer = 'asvetlov'
components = ['asyncio']
creation = <Date 2019-06-14.10:29:49.113>
creator = 'asvetlov'
dependencies = []
files = []
hgrepos = []
issue_num = 37280
keywords = ['patch']
message_count = 1.0
messages = ['345574']
nosy_count = 2.0
nosy_names = ['asvetlov', 'yselivanov']
pr_nums = ['14076', '14102', '14101']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue37280'
versions = ['Python 3.7', 'Python 3.8', 'Python 3.9']

[asvetlov]

We use thread pool executor for loop.sock_sendfile(), there is no reason to call blocking code for loop.sendfile()