Warn or fix whitespace in variable names

Continuing the discussion from here: https://github.com/rocky/python-uncompyle6/issues/312

@rocky directed me to post here instead.

It's possible to easily fool a lot of decompilers by renaming variables to names containing whitespace. For example, running this little script:

$ git clone https://github.com/ZetaTwo/python-obfuscator
$ cd python-obfuscator
$ ./test.sh

Will demonstrate how uncompyle6 can be tricked into decompiling the bytecode into valid code that does not reflect the actual bytecode.

My proposal is to add a small check that will at least warn about the presence of variables with invalid names and possinly even replace the invalid characters with for example "_".

I'm willing to make a PR with these changes but I would first just like to hear if I'm in the right place and if you have opinions on where/how this should be implemented?

Start with the linked discussion and reproduce the behavior using the python-obfuscator repository and its ./test.sh script. Then trace how python-xdis handles variable names during bytecode disassembly; done means invalid whitespace-containing names are warned about or replaced, with the obfuscator example no longer producing misleading output.