windows: fix PYTHON environment variable usage

- (README.md) The PYTHON environment variable should point to python.exe
not the python directory.

- (vcvars.bat) The PYTHON environment variable should be quoted on
expansion in case there's spaces in the path.

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The PYTHON environment variable should be quoted in case there's spaces in the path. For example assume `set PYTHON=C:\some path\python.exe` you'd want the command to be `"C:\some path\python.exe"` so PYTHON would need to be expanded in quotes `"%PYTHON%"`. This is typical in Windows because environment variables usually don't contain quotes and instead you're expected to quote on expansion.

Your contributing document says bugfixes go into the stable branch not master but I don't see a branch 'stable'.