Rule parsing in case of mixed ('\t' and ' ') indentation

[sharkovadarya]

rule foo:
    output: "file.txt"
    input: "input.txt"
    shell: "echo hello"

rule boo:
   input: ...

If the entire file is indented with tabs but a rule section, e.g. output, is indented with spaces, rule parsing fails up until the next rule (so, in this case, rule foo wouldn't be parsed correctly starting with input section, rule boo would be parsed correctly).

Python plugin reports an 'unexpected indent' error in such cases and suggests converting indentation, we could probably try to do the same.