Bump CI python to v3.8 (#186)

Github actions CI doens't support python 3.6 anymore, so we need to
start using python v3.8 for testing. Updated README to indicate that we test with python3.8 now and follow NEP29 from now on.

.github/workflows/pythonapp.yml

@@ -16,10 +16,10 @@ jobs:
 
     steps:
     - uses: actions/checkout@v2
-    - name: Set up Python 3.6
+    - name: Set up Python 3.8
       uses: actions/setup-python@v1
       with:
-        python-version: 3.6
+        python-version: 3.8
     - name: Install ncview
       run: sudo apt-get install ncview
     - name: Install dependencies

CHANGELOG.md

@@ -10,6 +10,11 @@
   formatted output files
   [\#182](https://github.com/EUREC4A-UK/lagtraj/pull/182/) @sjboeing
 
+*maintenance*
+
+- switch to using python v3.8 for continuous integration
+  [\#186](https://github.com/EUREC4A-UK/lagtraj/pull/186) @leifdenby
+
 
 ## [v0.1.1](https://github.com/EUREC4A-UK/lagtraj/tree/v0.1.1)
 

README.md

@@ -37,8 +37,10 @@ $> python -m pip install lagtraj
 *NOTE: if you are intending to modify `lagtraj` yourself you should check out
 the [development notes](docs/developing.md).*
 
-`lagtraj` requires Python 3 and is tested with `python3.6` but later
-versions should work too.
+`lagtraj` requires Python 3 and is developed and tested with `python3.8` (in
+that we aim to follow the recommendations of
+[NEP29](https://numpy.org/neps/nep-0029-deprecation_policy.html)) but later
+versions should work too (it may work with earlier versions too).
 
 Once installed all `lagtraj`'s commands are available from any directory
 and the follow the pattern