charm-tools: 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]

    clang -Wno-unused-result -Wsign-compare -Wunreachable-code -fno-common -dynamic -DNDEBUG -g -fwrapv -O3 -Wall -isysroot /Library/Developer/CommandLineTools/SDKs/MacOSX10.15.sdk -I/Library/Developer/CommandLineTools/SDKs/MacOSX10.15.sdk/usr/include -I/Library/Developer/CommandLineTools/SDKs/MacOSX10.15.sdk/System/Library/Frameworks/Tk.framework/Versions/8.5/Headers -I/usr/local/include -I/usr/local/opt/openssl@1.1/include -I/usr/local/opt/sqlite/include -I/usr/local/Cellar/charm-tools/2.7.8_1/libexec/include -I/usr/local/Cellar/python@3.9/3.9.0/Frameworks/Python.framework/Versions/3.9/include/python3.9 -c ext/_yaml.c -o build/temp.macosx-10.15-x86_64-3.9/ext/_yaml.o
    ext/_yaml.c:25698:26: error: no member named 'tp_print' in 'struct _typeobject'
      __pyx_type_5_yaml_Mark.tp_print = 0;
      ~~~~~~~~~~~~~~~~~~~~~~ ^
    ext/_yaml.c:25718:29: error: no member named 'tp_print' in 'struct _typeobject'
      __pyx_type_5_yaml_CParser.tp_print = 0;
      ~~~~~~~~~~~~~~~~~~~~~~~~~ ^
    ext/_yaml.c:25732:30: error: no member named 'tp_print' in 'struct _typeobject'
      __pyx_type_5_yaml_CEmitter.tp_print = 0;
      ~~~~~~~~~~~~~~~~~~~~~~~~~~ ^

[fxcoudert]

Reported to upstream: juju/charm-tools#586

[fxcoudert]

No news from upstream, we'll migrate when they fix it. Closing.