This program takes input.txt
It has unwanted (1234567890) and so on.

It only saves these letters:
A B C D E F G H I J K L M N O P Q R S T U V W X Y Z a b c d e f g h i j k l m n o p q r s t u v w x y z

(space button)

(enter button) and (tab button) is converted to (space button)

Now collapse consecutive whitespaces into one space.

The others are removed.

Then the output.txt is created.

Try this, untested and inefficient due to re but it should work:

import re

with open("input.txt", "r") as f:
    txt = f.read()

txt = re.sub(r"^A-Za-z]", '', txt)
txt = re.sub(r"\s+", " ", txt)

with open("output.txt", "w") as f:
    txt = f.write()