fix_import (2to3 fixer) is not case-sensitive

[progval]

BPO	18477
Nosy	@benjaminp, @progval
Superseder	bpo-45544: Close 2to3 issues and list them here
Files	fix_case-sensitivity_of_fix-import.patch: Patch to fix case-sentitivity of fix_import on Windows

^{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 = <Date 2021-10-20.22:59:08.762>
created_at = <Date 2013-07-16.21:10:50.155>
labels = ['type-bug', 'expert-2to3', 'OS-windows']
title = 'fix_import (2to3 fixer) is not case-sensitive'
updated_at = <Date 2021-10-20.22:59:08.762>
user = 'https://github.com/ProgVal'

bugs.python.org fields:

activity = <Date 2021-10-20.22:59:08.762>
actor = 'iritkatriel'
assignee = 'none'
closed = True
closed_date = <Date 2021-10-20.22:59:08.762>
closer = 'iritkatriel'
components = ['Windows', '2to3 (2.x to 3.x conversion tool)']
creation = <Date 2013-07-16.21:10:50.155>
creator = 'Valentin.Lorentz'
dependencies = []
files = ['30942']
hgrepos = []
issue_num = 18477
keywords = ['patch']
message_count = 2.0
messages = ['193193', '224357']
nosy_count = 2.0
nosy_names = ['benjamin.peterson', 'Valentin.Lorentz']
pr_nums = []
priority = 'normal'
resolution = 'wont fix'
stage = 'resolved'
status = 'closed'
superseder = '45544'
type = 'behavior'
url = 'https://bugs.python.org/issue18477'
versions = ['Python 3.4', 'Python 3.5']

[progval]

In a project, I have a package with a module named Socket.py, and the __init__.py imports the "socket" module (from the standard Python lib).
However, when fix_import went over it _on Windows_, it converted "import socket" to "from . import Socket".
I also had this issue from a package containing a subpackage named "Math", and a __init__.py importing "math" (from the standard Python lib).

This issue is caused by using os.path.exists(), which is case-insensitive on Windows. However, PEP-235 says imports are case-sensitive.

I'm including a patch I have wrote for this.

[BreamoreBoy]

Can we set the stage to patch review please.