Write a Python function count$\_$letters$($file$)$ for counting letters in a text file.

$$ Input: name of file relative to the present working directory pwd $($see below$),$ e$.$g$.$ count$\_$letters$(\text{'}$frost$.$txt$\text{'})$ if 'frost.txt$\text{'}$ is in the pwd

$$ Output: return the dictionary of letter:count pairs. Only include letters present in the file.

$$ Use a dictionary to hold a mapping between a letter and its number of occurrences.

$$ Ignore characters and symbols that are not standard ascii characters $($only use characters that appear in string.ascii$\_$lowercase$)$

$$ Ignore case; e$.$g$.$ consider 'Treat' as having $2\text{'}$t$\text{'}$s instead of $1$ T and $1$ t$.$

$$ Reading files $-$ You can use open to open a file. It creates an iterator that goes through the file, line by line.