You could use itertools.product:
print([''.join(x) for x in itertools.product('abcABC123', repeat=3)])
['aaa',
'aab',
'aac',
'aaA',
'aaB',
'aaC',
'aa1',
'aa2',
'aa3',
'aba',
...
Just add the remaining characters you need to the input string. You can use the constants from the strings
module for this.
Be aware that this quickly grows. ;)