fix indentation error with starting comment - #14721

closes #14242

This happens when executing something like:

    # comment
print("hello")

for i in range(7):
    print(i)
print("goodbye")

for some reason this is treated wrongly.
We have to explicitly check for starting # here.

Checking for multiline comments or single line strings with ' or " is not needed as cpython does not render such input valid as well.

I also experimented with some regex like this: ^[ \t]+(?=[^'"#\t ]) to check for existance of #, ' or ", though I don't think this is helpful here.

The issue is especially weird, as the typical error message is

  Cell In[1], line 5
    print("goodbye")
    ^
IndentationError: expected an indented block after 'for' statement on line 4

while the error is in the first line..