Use generator instead of list expand or add method

[jmishra01]

We can use generator to create pretty string of Tree in place of list. This PR reduce memory usage.

[erezsh]

I like this change!

Review: You can turn a few more of the "yield"s to "yield from"s.

[jmishra01]

@erezsh Done!

[erezsh]

What I meant is that you don't need any for loop..

yield from (i for i in ('\t', '%s' % (self.children[0],), '\n'))

can be

yield from ('\t', '%s' % (self.children[0],), '\n')

etc.

Although this particular expression can be rewritten as a single string format.

Give it another try if you like. Otherwise no problem, I'll do the finishing touches myself.

[jmishra01]

Thank you for review, I change few lines, use yield with f-string method.

[erezsh]

Thanks!