sceptre: migrate to python@3.9

[fxcoudert]

As part of the Python 3.9 migration (#62201).

This formula is independent from the all other Python formulas (if I didn't screw up my script or my logic)

Do not merge before the next Brew tag ships, expected on Monday 2020-10-12

[fxcoudert]

/usr/local/Cellar/sceptre/2.4.0_1/libexec/lib/python3.9/site-packages/networkx/convert_matrix.py:566: SyntaxWarning: "is" with a literal. Did you mean "=="?
  if python_type is 'void':
Traceback (most recent call last):
  File "/usr/local/Cellar/sceptre/2.4.0_1/bin/sceptre", line 33, in <module>
    sys.exit(load_entry_point('sceptre==2.4.0', 'console_scripts', 'sceptre')())
  File "/usr/local/Cellar/sceptre/2.4.0_1/bin/sceptre", line 25, in importlib_load_entry_point
    return next(matches).load()
  File "/usr/local/Cellar/python@3.9/3.9.0/Frameworks/Python.framework/Versions/3.9/lib/python3.9/importlib/metadata.py", line 77, in load
    module = import_module(match.group('module'))
  File "/usr/local/Cellar/python@3.9/3.9.0/Frameworks/Python.framework/Versions/3.9/lib/python3.9/importlib/__init__.py", line 127, in import_module
    return _bootstrap._gcd_import(name[level:], package, level)
  File "<frozen importlib._bootstrap>", line 1030, in _gcd_import
  File "<frozen importlib._bootstrap>", line 1007, in _find_and_load
  File "<frozen importlib._bootstrap>", line 986, in _find_and_load_unlocked
  File "<frozen importlib._bootstrap>", line 680, in _load_unlocked
  File "<frozen importlib._bootstrap_external>", line 790, in exec_module
  File "<frozen importlib._bootstrap>", line 228, in _call_with_frames_removed
  File "/usr/local/Cellar/sceptre/2.4.0_1/libexec/lib/python3.9/site-packages/sceptre/cli/__init__.py", line 19, in <module>
    from sceptre.cli.create import create_command
  File "/usr/local/Cellar/sceptre/2.4.0_1/libexec/lib/python3.9/site-packages/sceptre/cli/create.py", line 5, in <module>
    from sceptre.plan.plan import SceptrePlan
  File "/usr/local/Cellar/sceptre/2.4.0_1/libexec/lib/python3.9/site-packages/sceptre/plan/plan.py", line 12, in <module>
    from sceptre.config.graph import StackGraph
  File "/usr/local/Cellar/sceptre/2.4.0_1/libexec/lib/python3.9/site-packages/sceptre/config/graph.py", line 11, in <module>
    import networkx as nx
  File "/usr/local/Cellar/sceptre/2.4.0_1/libexec/lib/python3.9/site-packages/networkx/__init__.py", line 114, in <module>
    import networkx.generators
  File "/usr/local/Cellar/sceptre/2.4.0_1/libexec/lib/python3.9/site-packages/networkx/generators/__init__.py", line 14, in <module>
    from networkx.generators.intersection import *
  File "/usr/local/Cellar/sceptre/2.4.0_1/libexec/lib/python3.9/site-packages/networkx/generators/intersection.py", line 13, in <module>
    from networkx.algorithms import bipartite
  File "/usr/local/Cellar/sceptre/2.4.0_1/libexec/lib/python3.9/site-packages/networkx/algorithms/__init__.py", line 16, in <module>
    from networkx.algorithms.dag import *
  File "/usr/local/Cellar/sceptre/2.4.0_1/libexec/lib/python3.9/site-packages/networkx/algorithms/dag.py", line 23, in <module>
    from fractions import gcd
ImportError: cannot import name 'gcd' from 'fractions' (/usr/local/Cellar/python@3.9/3.9.0/Frameworks/Python.framework/Versions/3.9/lib/python3.9/fractions.py)

[fxcoudert]

Upstream report Sceptre/sceptre#942

[fxcoudert]

Closing, will reopen once upstream has fixed this.