python: backport 'drop Python3.6 workarounds'

python: backport 'drop Python3.6 workarounds'

Now that the minimum version is 3.7, drop some of the 3.6-specific hacks
we've been carrying. A single remaining compatibility hack concerning
3.6's lack of @asynccontextmanager is addressed in the following commit.

Signed-off-by: John Snow <jsnow@redhat.com>
cherry picked from commit python-qemu-qmp@3e8e34e594cfc6b707e6f67959166acde4b421b8
Signed-off-by: John Snow <jsnow@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
(cherry picked from commit f9d2e0a3bd7ba2a693a892881f91cf53fa90cc71)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>