bumped minimum Python version to 3.9 and bumped version to 0.20.0

Author: dave-doty

.github/workflows/run_unit_tests.yml

@@ -8,7 +8,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        python-version: [ 3.7, 3.8, 3.9, "3.10", "3.11", "3.12" ]
+        python-version: [ "3.9", "3.10", "3.11", "3.12", "3.13" ]
 
     steps:
       - uses: actions/checkout@v2

README.md

@@ -65,7 +65,7 @@ pip install scadnano
 Read below for troubleshooting suggestions if that didn't work.
 
 ### Getting Python
-The scadnano Python package requires Python version 3.7 or later.
+The scadnano Python package requires Python version 3.9 or later.
 
 To check your current version of Python, open a command line and type
 
@@ -79,7 +79,7 @@ If it is version 2.7 or below, type
 python3 --version
 ```
 
-If that fails, or reports Python version 3.6 or below, you will have to install a later version of Python. Follow [this link](https://www.python.org/downloads/) to install Python. You may also use an alternative Python distribution, such as [Anaconda](https://www.anaconda.com/products/individual#Downloads).
+If that fails, or reports Python version 3.8 or below, you will have to install a later version of Python. Follow [this link](https://www.python.org/downloads/) to install Python. You may also use an alternative Python distribution, such as [Anaconda](https://www.anaconda.com/products/individual#Downloads).
 
 
 
@@ -123,17 +123,11 @@ Once Python is installed, there are two ways you can install the scadnano Python
     ```
     pip 19.3.1 from ...lib\site-packages\pip (python 3.8)
     ```
-    If the version of Python at the end is Python 3.7 or higher, you are good. If it is version 2.7 or lower, type
+    If the version of Python at the end is Python 3.9 or higher, you are good. If it is version 2.7 or lower, type
     ```
     pip3 --version
     ```
-    If that works and shows Python 3.7 or higher, you are good, but you should type `pip3` in the subsequent instructions instead of `pip`.
-    
-    If it shows Python 3.6, install the [dataclasses backport module](https://pypi.org/project/dataclasses/) via
-    ```
-    pip install dataclasses
-    ```
-    If it shows Python 3.5 or lower, then you will need to upgrade your Python version (recommended Python 3.7 or higher).
+    If that works and shows Python 3.9 or higher, you are good, but you should type `pip3` in the subsequent instructions instead of `pip`.
 
     
 2. download

environment.yml

@@ -3,7 +3,7 @@ name: scadnano-test
 channels:
   - defaults
 dependencies:
-  - python=3.7
+  - python=3.9
   - pip
   - pip:
     - sphinx

scadnano/scadnano.py

@@ -54,7 +54,7 @@
 # needed to use forward annotations: https://docs.python.org/3/whatsnew/3.7.html#whatsnew37-pep563
 from __future__ import annotations
 
-__version__ = "0.19.5"  # version line; WARNING: do not remove or change this line or comment
+__version__ = "0.20.0"  # version line; WARNING: do not remove or change this line or comment
 
 import collections
 import dataclasses
@@ -2950,7 +2950,7 @@ class StrandBuilder:
 
     This is also useful if you create strands in a loop and want to call some of the methods conditionally.
     For example to create many strands where only the first has a particular modification:
-    
+
     .. code-block:: Python
 
         for i in range(n):