Update minimum Python version to 3.6.2 (#2093)

* Correct minimum version

see #2092 

While this is needed because of an import just for typing, I see no reason not to bump the minimum version since this is a minor version difference since this is several minor version behind the latest 3.6, and there have been both security and performance improvements since.

That said, we need to be testing on our lowest supported version to ensure we don't have this happen again, right now our tests run on whatever Travis grabs for 3.6, which I assume is 3.6.6, but could be wrong.

* Update other mentions of min version to 3.6.2

.github/CONTRIBUTING.md

@@ -53,7 +53,7 @@ Red's repository is configured to follow a particular development workflow, usin
 
 ### 4.1 Setting up your development environment
 The following requirements must be installed prior to setting up:
- - Python 3.6
+ - Python 3.6.2 or greater
  - git
  - pip
  - pipenv
@@ -79,7 +79,7 @@ Note: If you haven't used `pipenv` before but are comfortable with virtualenvs,
 We've recently started using [tox](https://github.com/tox-dev/tox) to run all of our tests. It's extremely simple to use, and if you followed the previous section correctly, it is already installed to your virtual environment.
 
 Currently, tox does the following, creating its own virtual environments for each stage:
-- Runs all of our unit tests with [pytest](https://github.com/pytest-dev/pytest) on python 3.6 (test environment `py36`)
+- Runs all of our unit tests with [pytest](https://github.com/pytest-dev/pytest) on python 3.6 and 3.7 (test environments `py36` and `py37`)
 - Ensures documentation builds without warnings, and all hyperlinks have a valid destination (test environment `docs`)
 - Ensures that the code meets our style guide with [black](https://github.com/ambv/black) (test environment `style`)