pdb: Add step / next count arguments

GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2017-06-26.10:10:13.014>
labels = ['3.7', 'type-feature', 'library']
title = 'pdb: Add step / next count arguments'
updated_at = <Date 2017-08-26.19:29:59.821>
user = 'https://github.com/mlouielu'

bugs.python.org fields:

activity = <Date 2017-08-26.19:29:59.821>
actor = 'barry'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Library (Lib)']
creation = <Date 2017-06-26.10:10:13.014>
creator = 'louielu'
dependencies = []
files = []
hgrepos = []
issue_num = 30761
keywords = []
message_count = 4.0
messages = ['296869', '296871', '296906', '296930']
nosy_count = 5.0
nosy_names = ['barry', 'rhettinger', 'vstinner', 'xdegaye', 'louielu']
pr_nums = ['2399']
priority = 'normal'
resolution = None
stage = None
status = 'open'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue30761'
versions = ['Python 3.7']