python 3.6 reached end of life on 2021-12-23

Committed via https://github.com/asottile/all-repos

Committed via https://github.com/asottile/all-repos

Fixes #300

Do a straight test of detecting a real merge conflict as generated by git.
Test artificial conflict detection while pending merge without a real conflict.
Test artificial non-conflict non-detection in a resolved merge conflict.
Rename test_does_not_care... function to reflect what we want to care about.
Rename is_in_merge_conflict to is_in_merge since that is what it checks.

Several markup formats, such as Markdown or Re(Structured)Text
can format titles as text with '=' characters as double underlining,
like this:
```
My Page Title
=============

Lorem ipsum...
```

Rather that considering any line starting with seven '=' as a conflict marker,
require a space (or line-ending newline) after the equals.
This could still create a false positive for a seven character title,
like "Problem", but the markup formats generally allow extra '=' characters,
so by formatting the text like this:
```
Problem
========

Not...
```
these pre-commit warnings can be avoided.

Also updates the tests to add newlines for more realistic conflict files
(while a file might not end with a newline, conflict markers will).
Prevent false negative on test_does_not_care_when_not_in_a_conflict()
by making sure that README.md contains a line identical to a conflict string
(exactly seven '=' followed by a newline).