Update dev guide with correct minimum Python version

DEVELOPMENT_GUIDE.md

@@ -20,8 +20,8 @@ Gitpod is free for 50 hours per month - make sure to stop your workspace when yo
 ## Environment Setup
 ### 1. Prerequisites (Python Virtual Environment)
 
-AWS SAM CLI is mainly written in Python 3 and we support Python 3.7 and 3.8.
-So having a Python environment with aforementioned versions is required.
+AWS SAM CLI is mainly written in Python 3 and we support Python 3.8 and above.
+So, having a Python environment with this version is required.
 
 Having a dedicated Python virtual environment ensures it won't "pollute" or get "polluted" 
 by other python packages. Here we introduce two ways of setting up a Python virtual environment:
@@ -163,26 +163,27 @@ contribute to the repository, there are a few more things to consider.
 
 ### Make Sure AWS SAM CLI Work in Multiple Python Versions
 
-We support 3.7 and 3.8 versions. Our CI/CD pipeline is setup to run
+We support version 3.8 and above. Our CI/CD pipeline is setup to run
 unit tests against all Python versions. So make sure you test it
 with all versions before sending a Pull Request.
-See [Unit testing with multiple Python versions](#unit-testing-with-multiple-python-versions-optional).
+See [Unit testing with multiple Python versions](#unit-testing-with-multiple-python-versions-optional). 
+This is most important if you are developing in a Python version greater than the minimum supported version (currently 3.8), as any new features released in 3.9+ will not work.
 
 If you chose to use `pyenv` in the previous session, setting up a 
 different Python version should be easy:
 
-(assuming you are in virtual environment `samcli38`)
+(assuming you are in virtual environment named `samcli39` with Python version 3.9.x)
 
 ```sh
-# Your shell now should looks like "(samcli38) $"
-pyenv deactivate samcli38  # "(samcli38)" will disappear
-pyenv install 3.7.10  # one time setup
-pyenv virtualenv 3.7.10 samcli37  # one time setup
-pyenv activate samcli37
-# Your shell now should looks like "(samcli37) $"
+# Your shell now should look like "(samcli39) $"
+pyenv deactivate samcli39  # "(samcli39)" will disappear
+pyenv install 3.8.9  # one time setup
+pyenv virtualenv 3.8.9 samcli38  # one time setup
+pyenv activate samcli38
+# Your shell now should look like "(samcli38) $"
 
 # You can verify the version of Python
-python --version  # Python 3.7.10
+python --version  # Python 3.8.9
 
 make init  # one time setup, this will put a file `samdev` available in $PATH
 ```
@@ -255,9 +256,9 @@ We also suggest to run `make pr` or `./Make -pr` in all Python versions.
 #### Unit Testing with Multiple Python Versions (Optional)
 
 Currently, SAM CLI only supports Python3 versions (see setup.py for exact versions). For the most
-part, code that works in Python3.7 will work in Python3.8. You only run into problems if you are
-trying to use features released in a higher version (for example features introduced into Python3.8
-will not work in Python3.7). If you want to test in many versions, you can create a virtualenv for
+part, code that works in Python3.8 will work in Python3.9. You only run into problems if you are
+trying to use features released in a higher version (for example features introduced into Python3.9
+will not work in Python3.8). If you want to test in many versions, you can create a virtualenv for
 each version and flip between them (sourcing the activate script). Typically, we run all tests in
 one python version locally and then have our ci (appveyor) run all supported versions.