remove unneeded handling of '.' and '..' from patlib.Path.iterdir()

GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2020-06-27.13:07:24.566>
labels = ['library']
title = "remove unneeded handling of '.' and '..' from patlib.Path.iterdir()"
updated_at = <Date 2020-06-27.13:09:15.962>
user = 'https://github.com/sir-sigurd'

bugs.python.org fields:

activity = <Date 2020-06-27.13:09:15.962>
actor = 'sir-sigurd'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Library (Lib)']
creation = <Date 2020-06-27.13:07:24.566>
creator = 'sir-sigurd'
dependencies = []
files = []
hgrepos = []
issue_num = 41141
keywords = ['patch']
message_count = 1.0
messages = ['372465']
nosy_count = 1.0
nosy_names = ['sir-sigurd']
pr_nums = ['21179']
priority = 'normal'
resolution = None
stage = 'patch review'
status = 'open'
superseder = None
type = None
url = 'https://bugs.python.org/issue41141'
versions = []