gzip._GzipReader should only throw BadGzipFile errors

[rhpvorderman]

BPO	43621
Nosy	@rhpvorderman

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2021-03-25.08:24:44.466>
labels = ['type-bug', '3.8', '3.9', '3.10', '3.7', 'library']
title = 'gzip._GzipReader should only throw BadGzipFile errors'
updated_at = <Date 2021-03-25.08:25:21.785>
user = 'https://github.com/rhpvorderman'

bugs.python.org fields:

activity = <Date 2021-03-25.08:25:21.785>
actor = 'rhpvorderman'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Library (Lib)']
creation = <Date 2021-03-25.08:24:44.466>
creator = 'rhpvorderman'
dependencies = []
files = []
hgrepos = []
issue_num = 43621
keywords = []
message_count = 1.0
messages = ['389494']
nosy_count = 1.0
nosy_names = ['rhpvorderman']
pr_nums = []
priority = 'normal'
resolution = None
stage = None
status = 'open'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue43621'
versions = ['Python 3.6', 'Python 3.7', 'Python 3.8', 'Python 3.9', 'Python 3.10']

[rhpvorderman]

This is properly documented: https://docs.python.org/3/library/gzip.html#gzip.BadGzipFile .

It now hrows EOFErrors when a stream is truncated. But this means that upstream both BadGzipFile and EOFError need to be catched in the exception handling when opening a gzip file for reading. When a gzip file is truncated it is also a "bad gzip file" in my opinion, so there is no reason to have an extra class of errors.
Also it throws zlib.error's when zlib craches for some reason. This means there is some corruption in the raw deflate block. Well that means it is a "bad gzip file" as well and the error message should reflect that.

This won't break people's code. If they are already catching EOFError zlib.error and BadGzipFile it changes nothing. If they only catch BadGzipFile, they will have less annoying errors that pop through.

I can make the PR, but of course not without any feedback. I am curious what other people think.