asyncio: Move loop attribute to _FlowControlMixin

Move the _loop attribute from the constructor of _SelectorTransport,
_ProactorBasePipeTransport and _UnixWritePipeTransport classes to the
constructor of the _FlowControlMixin class.

Add also an assertion to explicit that the parent class must ensure that the
loop is defined (not None)

--HG--
branch : 3.4
use pythontest.net for url fragment test

--HG--
branch : 3.2
extra : source : 603bd221527cb535ee0ef99d9f02351afa4feceb
merge 3.2

--HG--
branch : 3.3
merge 3.3

--HG--
branch : 3.4
remove requires_ssl decorator

--HG--
branch : 3.4
use pythontest.net for fragment redirection test

--HG--
branch : 3.4
Closes #22525: clarify documentation for ast.literal_eval().

--HG--
branch : 3.4
Closes #22784: fix test_asyncio when the ssl module isn't available

--HG--
branch : 3.4
test_httplib: use self-signed.pythontest.net for verification test with non-root-CA cert

--HG--
branch : 3.4
extra : rebase_source : a946d87db5cfb4849afbb5d0b6fd5448fffd1bad