Update Python requirement to >=3.8. (#8071)

Additional changes:
- Use mamba for CPU test on Jenkins.
- Cleanup CPU test dependencies.
- Restore some of the modin tests

.github/workflows/main.yml

@@ -136,7 +136,7 @@ jobs:
         submodules: 'true'
     - uses: actions/setup-python@v2
       with:
-        python-version: '3.7'
+        python-version: "3.8"
         architecture: 'x64'
     - name: Install Python packages
       run: |
@@ -155,7 +155,7 @@ jobs:
         submodules: 'true'
     - uses: actions/setup-python@v2
       with:
-        python-version: '3.7'
+        python-version: "3.8"
         architecture: 'x64'
     - name: Install Python packages
       run: |
@@ -173,7 +173,7 @@ jobs:
         submodules: 'true'
     - uses: actions/setup-python@v2
       with:
-        python-version: '3.7'
+        python-version: "3.8"
         architecture: 'x64'
     - name: Install system packages
       run: |
@@ -210,7 +210,7 @@ jobs:
         submodules: 'true'
     - uses: actions/setup-python@v2
       with:
-        python-version: '3.8'
+        python-version: "3.8"
         architecture: 'x64'
     - name: Install system packages
       run: |

.github/workflows/python_wheels.yml

@@ -20,7 +20,7 @@ jobs:
     - name: Setup Python
       uses: actions/setup-python@v2
       with:
-        python-version: '3.9'
+        python-version: "3.8"
     - name: Build wheels
       run: bash tests/ci_build/build_python_wheels.sh ${{ matrix.platform_id }} ${{ github.sha }}
     - name: Extract branch name

.github/workflows/r_tests.yml

@@ -97,7 +97,7 @@ jobs:
 
     - uses: actions/setup-python@v2
       with:
-        python-version: '3.7'
+        python-version: "3.8"
         architecture: 'x64'
 
     - name: Test R