Fix flake8 errors

[ovuolteenaho]

Assigning to Teemu for now, he can delegate this forward.

$ flake8
./idiokit/_selectloop.py:181:9: E722 do not use bare except'
./idiokit/idiokit.py:120:5: E722 do not use bare except'
./idiokit/idiokit.py:357:9: E722 do not use bare except'
./idiokit/select.py:14:9: E722 do not use bare except'
./idiokit/socket.py:339:5: E722 do not use bare except'
./idiokit/threadpool.py:88:13: E722 do not use bare except'
./idiokit/http/server.py:125:13: E722 do not use bare except'
./idiokit/http/server.py:637:9: E722 do not use bare except'
./idiokit/xmpp/muc.py:224:9: E722 do not use bare except'

These are soon ignored in tox.ini, so please remove E722 after fixing:

[flake8]
ignore = E501,E722