Incompatibility with Python 3.10

[stilllman]

With Python 3.10, installing or running rez generates ImportError of collections ABC:

Traceback (most recent call last):
  File "/home/luc/tools/install/rez/lib64/python3.10/site-packages/rez/util.py", line 158, in _atexit
    from rez.resolved_context import ResolvedContext
  File "/home/luc/tools/install/rez/lib64/python3.10/site-packages/rez/resolved_context.py", line 20, in <module>
    from rez.solver import SolverCallbackReturn
  File "/home/luc/tools/install/rez/lib64/python3.10/site-packages/rez/solver.py", line 41, in <module>
    from rez.vendor.sortedcontainers.sortedset import SortedSet
  File "/home/luc/tools/install/rez/lib64/python3.10/site-packages/rez/vendor/sortedcontainers/__init__.py", line 41, in <module>
    from .sortedlist import SortedList, SortedListWithKey
  File "/home/luc/tools/install/rez/lib64/python3.10/site-packages/rez/vendor/sortedcontainers/sortedlist.py", line 9, in <module>
    from collections import Sequence, MutableSequence
ImportError: cannot import name 'Sequence' from 'collections' (/usr/lib64/python3.10/collections/__init__.py)
Exception ignored in atexit callback: <function _atexit at 0x7f87816f6560>

This comes from imports in sortedcontainers, which could be updated to fix the issue.