Drop Python 3.6 support (#699)

Co-authored-by: Wei Ji <23487320+weiji14@users.noreply.github.com>

Author: seisman

.github/workflows/ci_tests.yaml

@@ -42,19 +42,8 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        python-version: [3.6, 3.7, 3.8, 3.9]
+        python-version: [3.7, 3.8, 3.9]
         os: [ubuntu-latest, macOS-latest, windows-latest]
-        # Run Python 3.6 tests on master branch only to conserve CI resources
-        isMaster:
-          - ${{ contains(github.ref, 'master') }}
-        exclude:
-          - isMaster: false  # i.e. on Pull Requests
-            python-version: 3.6
-          # exclude Windows + Python 3.6 due to issue #693
-          # Remove it when we drop Python 3.6 support
-          - isMaster: true
-            python-version: 3.6
-            os: windows-latest
     # environmental variables used in coverage
     env:
       OS: ${{ matrix.os }}

doc/install.rst

@@ -16,7 +16,8 @@ Installing
 Which Python?
 -------------
 
-You'll need **Python 3.6 or greater** to run PyGMT.
+You'll need **Python 3.7 or greater** to run PyGMT. Old Python versions may
+work, but there is no guarantee that PyGMT will work as expected with old versions.
 
 We recommend using the `Anaconda <https://www.anaconda.com/distribution>`__ Python
 distribution to ensure you have all dependencies installed and the ``conda``

setup.py

@@ -28,7 +28,6 @@
     "Intended Audience :: Education",
     "Topic :: Scientific/Engineering",
     "Topic :: Software Development :: Libraries",
-    "Programming Language :: Python :: 3.6",
     "Programming Language :: Python :: 3.7",
     "Programming Language :: Python :: 3.8",
     "Programming Language :: Python :: 3.9",