[py2py3] Fix pre-python2.6 idiom: use "isinstance()" instead of "type() == "

[mapellidario]

Impact of the new feature

Likely to involve a large portion of the codebase.

Is your feature request related to a problem? Please describe.

The "python3 compatibility report" section of the Jenkins report often suggests the following change, in order not to use a pre-py2.6 idiom:

- if type(variable) == mytype:
+ if isinstance(variable, mytype):

Since these are almost always safe changes, they can be tackled all at once.

Describe the solution you'd like

We should apply the changes suggested by the "python3 compatibility report" regarding pre-py2.6 idioms once and for all.

Additional context

This issue was suggested in the comments section of #10103