Drop support for Python 3.6 and use Python 3.9 by default. (#40)

* Drop support for Python 3.6 and use Python 3.9 by default.

Why?
- Python 3.6 will reach end of life soon (Dec' 2021)
- Python 3.9 is the latest version that qiskit-terra currently supports

* Update pylint to most recent version

* Remove unused import

.github/workflows/cron-other.yml

@@ -20,7 +20,7 @@ jobs:
     runs-on: ${{ matrix.os }}
     strategy:
       matrix:
-        python-version: [3.6, 3.7, 3.8, 3.9]
+        python-version: [3.7, 3.8, 3.9]
         os: ["windows-latest", "ubuntu-latest"]
     env:
       QISKIT_IBM_RUNTIME_API_TOKEN: ${{ secrets.QISKIT_IBM_RUNTIME_API_TOKEN }}
@@ -49,7 +49,7 @@ jobs:
     runs-on: ${{ matrix.os }}
     strategy:
       matrix:
-        python-version: [3.6, 3.7, 3.8, 3.9]
+        python-version: [3.7, 3.8, 3.9]
         os: ["windows-latest", "ubuntu-latest"]
     env:
       QISKIT_IBM_RUNTIME_API_TOKEN: ${{ secrets.QISKIT_IBM_RUNTIME_API_TOKEN }}
@@ -77,7 +77,7 @@ jobs:
     runs-on: ${{ matrix.os }}
     strategy:
       matrix:
-        python-version: [3.6, 3.7, 3.8, 3.9]
+        python-version: [3.7, 3.8, 3.9]
         os: ["windows-latest", "ubuntu-latest"]
     env:
       QISKIT_IBM_RUNTIME_API_TOKEN: ${{ secrets.QISKIT_IBM_RUNTIME_API_TOKEN }}

.github/workflows/cron-prod.yml

@@ -15,10 +15,10 @@ jobs:
       QISKIT_IN_PARALLEL: True
     steps:
       - uses: actions/checkout@v2
-      - name: Set up Python 3.8
+      - name: Set up Python 3.9
         uses: actions/setup-python@v2
         with:
-          python-version: 3.8
+          python-version: 3.9
       - name: Install Deps
         run: |
           python -m pip install --upgrade pip

.github/workflows/cron-slow.yml

@@ -29,10 +29,10 @@ jobs:
       QISKIT_IN_PARALLEL: True
     steps:
       - uses: actions/checkout@v2
-      - name: Set up Python 3.6
+      - name: Set up Python 3.9
         uses: actions/setup-python@v2
         with:
-          python-version: 3.6
+          python-version: 3.9
       - name: Install Deps
         run: |
           python -m pip install --upgrade pip
@@ -54,10 +54,10 @@ jobs:
       QISKIT_IN_PARALLEL: True
     steps:
       - uses: actions/checkout@v2
-      - name: Set up Python 3.8
+      - name: Set up Python 3.9
         uses: actions/setup-python@v2
         with:
-          python-version: 3.8
+          python-version: 3.9
       - name: Install Deps
         run: |
           python -m pip install --upgrade pip

.github/workflows/cron-staging.yml

@@ -28,10 +28,10 @@ jobs:
       QISKIT_IN_PARALLEL: True
     steps:
       - uses: actions/checkout@v2
-      - name: Set up Python 3.8
+      - name: Set up Python 3.9
         uses: actions/setup-python@v2
         with:
-          python-version: 3.8
+          python-version: 3.9
       - name: Install Deps
         run: |
           python -m pip install --upgrade pip

.github/workflows/deploy.yml

@@ -22,7 +22,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        python-version: [3.7]
+        python-version: [3.9]
     steps:
       - uses: actions/checkout@v2
       - uses: actions/setup-python@v2

.github/workflows/docs.yml

@@ -15,7 +15,7 @@ jobs:
     - name: Set up Python
       uses: actions/setup-python@v2
       with:
-        python-version: '3.7.8'
+        python-version: '3.9.9'
     - name: Install dependencies
       run: |
         python -m pip install --upgrade pip

.github/workflows/main.yml

@@ -18,10 +18,10 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v2
-      - name: Set up Python 3.7
+      - name: Set up Python 3.9
         uses: actions/setup-python@v2
         with:
-          python-version: 3.7
+          python-version: 3.9
       - name: Install Deps
         run: |
           python -m pip install --upgrade pip
@@ -39,10 +39,10 @@ jobs:
       - uses: actions/checkout@v2
         with:
           fetch-depth: 0
-      - name: Set up Python 3.7
+      - name: Set up Python 3.9
         uses: actions/setup-python@v2
         with:
-          python-version: 3.7
+          python-version: 3.9
       - name: Install Deps
         run: |
           python -m pip install --upgrade pip
@@ -61,7 +61,7 @@ jobs:
     runs-on: macOS-latest
     strategy:
       matrix:
-        python-version: [3.6, 3.7, 3.8, 3.9]
+        python-version: [3.7, 3.8, 3.9]
     env:
       QISKIT_IBM_RUNTIME_API_TOKEN: ${{ secrets.QISKIT_IBM_RUNTIME_API_TOKEN }}
       QISKIT_IBM_RUNTIME_API_URL: ${{ secrets.QISKIT_IBM_RUNTIME_API_URL }}

.github/workflows/pr.yml

@@ -18,7 +18,7 @@ jobs:
     runs-on: ${{ matrix.os }}
     strategy:
       matrix:
-        python-version: [3.6, 3.7, 3.8, 3.9]
+        python-version: [3.7, 3.8, 3.9]
         os: ["macOS-latest", "ubuntu-latest", "windows-latest"]
     env:
       QISKIT_IBM_RUNTIME_API_TOKEN: ${{ secrets.QISKIT_IBM_RUNTIME_API_TOKEN }}
@@ -47,10 +47,10 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v2
-      - name: Set up Python 3.7
+      - name: Set up Python 3.9
         uses: actions/setup-python@v2
         with:
-          python-version: 3.7
+          python-version: 3.9
       - name: Install Deps
         run: |
           python -m pip install --upgrade pip
@@ -68,10 +68,10 @@ jobs:
       - uses: actions/checkout@v2
         with:
           fetch-depth: 0
-      - name: Set up Python 3.7
+      - name: Set up Python 3.9
         uses: actions/setup-python@v2
         with:
-          python-version: 3.7
+          python-version: 3.9
       - name: Install Deps
         run: |
           python -m pip install --upgrade pip