dynamic-import: use sysstr for importing extension and others This logic is used by extensions, and python hooks and merge-tools. All this logic eventually deals with native string (unicode in Python 3). This patch makes it handle `str` directly instead of relying on some pycompat low lever layer to do the conversion at the last minutes. We adjust the Python version filtering of a test as the output seems to be present with Python 3.7 too.

extension: check the command attributes using `sysstr` Since we are checking attributes, lets use the native representation instead of bytes.

extension: access special module members using sysstr These extensions variables and mapping are module attributes so they should be dealt with unicode `str` in Python 3. We move the part that deal with reading theses variable and checking their validity to use unicode `str` string.

sshpeer: convert command name to sysstr before accessing method Method name are `str` so we should pass them as `str`.

localrepo: purge filecache attribute using there unicode name This could be better, but that's a good step.

fastannotate: use sysstr to check for attribute presence We do not need bytes here.

fastannotate: use sysstr to deal with some attributes We are now Python3 only and attribute are always unicode string. So no needs to managed them using bytes.

hgweb: use sysstr to set attribute on diff option Attribute identifier should be `str` not `bytes`.

hgweb: use sysstr to get attribute on diff option Attribute identifier should be `str` not `bytes`.

diff-option: move attributes handling to sysstr Attributes are `str` and should be handled as such.