Drop support of Python 3.6

It reached its end of life in December 2021.

Author: dbaty

.github/workflows/ci.yaml

@@ -10,7 +10,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        python-version: [3.6, 3.7, 3.8, 3.9, "3.10"]
+        python-version: [3.7, 3.8, 3.9, "3.10"]
 
     steps:
     - uses: actions/checkout@v2

docs/getting_started.rst

@@ -1,7 +1,7 @@
 Getting started
 ===============
 
-You must have Python 3.6 or later, and a relatively recent version of
+You must have Python 3.7 or later, and a relatively recent version of
 Git. Git 2.30 is known to work, but older versions will most likely
 work as well.
 

setup.cfg

@@ -17,7 +17,6 @@ classifiers =
     Programming Language :: Python
     Programming Language :: Python :: 3
     Programming Language :: Python :: 3 :: Only
-    Programming Language :: Python :: 3.6
     Programming Language :: Python :: 3.7
     Programming Language :: Python :: 3.8
     Programming Language :: Python :: 3.9
@@ -30,9 +29,8 @@ project_urls =
     Bug Tracker =  https://github.com/Polyconseil/cogite/issues
 
 [options]
-python_requires = >=3.6
+python_requires = >=3.7
 install_requires =
-    dataclasses; python_version<"3.7"
     importlib_metadata; python_version<"3.8"
     prompt_toolkit
     toml