Install mypy on direct CI jobs to run new tests

Also remove pytest-pep8 as it is obsolete and these CI jobs were not
in fact running pep8 linting anyway.

Make importing Literal compatible with Python 3.7 in pysam/libctabix.pyi
similarly to other pysam/*.pyi files.

.github/workflows/ci.yaml

@@ -28,7 +28,7 @@ jobs:
           python-version: ${{ matrix.python-version }}
 
       - name: Install prerequisite Python libraries
-        run:  pip install cython pytest pytest-pep8 setuptools
+        run:  pip install cython mypy pytest setuptools
 
       - name: Install Linux build prerequisites
         if:   runner.os == 'Linux'
@@ -79,7 +79,7 @@ jobs:
           python-version: ${{ matrix.python-version }}
 
       - name: Install prerequisite Python libraries
-        run:  pip install cython pytest pytest-pep8
+        run:  pip install cython pytest
 
       - name: Install build prerequisites
         if:   runner.os == 'Linux'

pysam/libctabix.pyi

@@ -1,4 +1,10 @@
-from typing import Optional, Literal, List, Any
+import sys
+from typing import Optional, List, Any
+
+if sys.version_info < (3, 8):
+    from typing_extensions import Literal
+else:
+    from typing import Literal
 
 from pysam.libchtslib import HTSFile