Add Python 3.8 to CI (#359)

* Add Python 3.8 to CI

* only oldest + latest
Co-authored-by: Farhad Ramezanghorbani <farhadrgh@users.noreply.github.com>

.github/workflows/deploy-docs.yml

@@ -11,7 +11,7 @@ jobs:
     strategy:
       max-parallel: 4
       matrix:
-        python-version: [3.7]
+        python-version: [3.8]
 
     steps:
     - uses: actions/checkout@v1

.github/workflows/deploy-pypi.yml

@@ -11,7 +11,7 @@ jobs:
     strategy:
       max-parallel: 4
       matrix:
-        python-version: [3.7]
+        python-version: [3.8]
 
     steps:
     - uses: actions/checkout@v1

.github/workflows/deploy-test-pypi.yml

@@ -15,7 +15,7 @@ jobs:
     strategy:
       max-parallel: 4
       matrix:
-        python-version: [3.7]
+        python-version: [3.8]
 
     steps:
     - uses: actions/checkout@v1

.github/workflows/docs.yml

@@ -15,7 +15,7 @@ jobs:
     strategy:
       max-parallel: 4
       matrix:
-        python-version: [3.6, 3.7]
+        python-version: [3.6, 3.8]
 
     steps:
     - uses: actions/checkout@v1

.github/workflows/flake8.yml

@@ -15,7 +15,7 @@ jobs:
     strategy:
       max-parallel: 4
       matrix:
-        python-version: [3.6, 3.7]
+        python-version: [3.6, 3.8]
 
     steps:
     - uses: actions/checkout@v1

.github/workflows/runnable_submodules.yml

@@ -15,7 +15,7 @@ jobs:
     strategy:
       max-parallel: 4
       matrix:
-        python-version: [3.6, 3.7]
+        python-version: [3.6, 3.8]
 
     steps:
     - uses: actions/checkout@v1

.github/workflows/tools.yml

@@ -15,7 +15,7 @@ jobs:
     strategy:
       max-parallel: 4
       matrix:
-        python-version: [3.6, 3.7]
+        python-version: [3.6, 3.8]
 
     steps:
     - uses: actions/checkout@v1

.github/workflows/unittest.yml

@@ -15,7 +15,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        python-version: [3.6, 3.7]
+        python-version: [3.6, 3.8]
         test-filenames: [
           test_aev.py, test_aev_benzene_md.py, test_aev_nist.py, test_aev_tripeptide_md.py,
           test_data_new.py, test_utils.py, test_ase.py, test_energies.py, test_periodic_table_indexing.py,