Does not work with Python 3.9

[fxcoudert]

Checklist

• [X] Confirmed this is an issue with charm-tools, not charmstore-client
• [X] Provide versions of tools used
• [X] Described the feature or ways to replicate the issue

What version am I running?

Trying to install charm-tools-2.7.8 on macOS 10.15.7 with Python 3.9 from Homebrew: https://github.com/Homebrew/homebrew-core/pull/62269/files

Issue/Feature

I expect/expected the following

I am running:

python3.9 -c import setuptools... --no-user-cfg install --prefix=/private/tmp/charm-tools--homebrew-virtualenv-20201007-45277-l62nlx/target/vendor --install-scripts=/private/tmp/charm-tools--homebrew-virtualenv-20201007-45277-l62nlx/target/vendor/bin --single-version-externally-managed --record=installed.txt

and expect installation to work.

What I got

The pyyaml version requested is too old, and does not support Python 3.9. I get the following error:

    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;
      ~~~~~~~~~~~~~~~~~~~~~~~~~~ ^