[test] Use pytest instead of nosetests (#482)

`nosetests` is no longer being maintained : https://github.com/nose-devs/nose/issues/1099
and will stop working in py 3.10 as can be seen in #480

.github/workflows/core.yml

@@ -23,11 +23,9 @@ jobs:
       uses: actions/setup-python@v2
       with:
         python-version: ${{ matrix.python-version }}
-    - name: Install nose
-      run: pip install nose
+    - name: Install pytest
+      run: pip install pytest
     - name: Run tests
       continue-on-error: False
-      env:
-        YTDL_TEST_SET: core
-      run: ./devscripts/run_tests.${{ matrix.run-tests-ext }}
+      run: ./devscripts/run_tests.${{ matrix.run-tests-ext }} core
   # Linter is in quick-test

.github/workflows/download.yml

@@ -21,10 +21,8 @@ jobs:
       uses: actions/setup-python@v2
       with:
         python-version: ${{ matrix.python-version }}
-    - name: Install nose
-      run: pip install nose
+    - name: Install pytest
+      run: pip install pytest
     - name: Run tests
       continue-on-error: true
-      env:
-        YTDL_TEST_SET: download
-      run: ./devscripts/run_tests.${{ matrix.run-tests-ext }}
+      run: ./devscripts/run_tests.${{ matrix.run-tests-ext }} download

.github/workflows/quick-test.yml

@@ -12,11 +12,9 @@ jobs:
       with:
         python-version: 3.9
     - name: Install test requirements
-      run: pip install nose pycryptodome
+      run: pip install pytest pycryptodome
     - name: Run tests
-      env:
-        YTDL_TEST_SET: core
-      run: ./devscripts/run_tests.sh
+      run: ./devscripts/run_tests.sh core
   flake8:
     name: Linter
     if: "!contains(github.event.head_commit.message, 'ci skip all')"
@@ -30,4 +28,4 @@ jobs:
     - name: Install flake8
       run: pip install flake8
     - name: Run flake8
-      run: flake8 .
+      run: flake8 .