Test errors on openSUSE

I have a few errors building on openSUSE Tumbleweed (py38 currently)
Packaging in progress at https://build.opensuse.org/package/show/home:jayvdb:py-new/python-libcst

There are tests in the sdist, but that failed due to missing test data.

The first of the remaining ones below is easy, related to using `sys.executable` instead of literal `python`. The others are more of a mystery at first glance.

```py
[ 118s] ======================================================================
[ 118s] ERROR: test_codemod_formatter_error_input (libcst.codemod.tests.test_codemod_cli.TestCodemodCLI)
[ 118s] ----------------------------------------------------------------------
[ 118s] Traceback (most recent call last):
[ 118s] File "/home/abuild/rpmbuild/BUILD/LibCST-0.3.7/libcst/codemod/tests/test_codemod_cli.py", line 16, in test_codemod_formatter_error_input
[ 118s] rlt = subprocess.run(
[ 118s] File "/usr/lib64/python3.8/subprocess.py", line 489, in run
[ 118s] with Popen(*popenargs, **kwargs) as process:
[ 118s] File "/usr/lib64/python3.8/subprocess.py", line 854, in __init__
[ 118s] self._execute_child(args, executable, preexec_fn, close_fds,
[ 118s] File "/usr/lib64/python3.8/subprocess.py", line 1702, in _execute_child
[ 118s] raise child_exception_type(errno_num, err_msg, err_filename)
[ 118s] FileNotFoundError: [Errno 2] No such file or directory: 'python'
[ 118s]
[ 118s] ======================================================================
[ 118s] FAIL: test_codegen_clean_matcher_classes (libcst.codegen.tests.test_codegen_clean.TestCodegenClean)
[ 118s] Verifies that codegen of matcher classes would not result in a
[ 118s] ----------------------------------------------------------------------
[ 118s] Traceback (most recent call last):
[ 118s] File "/home/abuild/rpmbuild/BUILD/LibCST-0.3.7/libcst/codegen/tests/test_codegen_clean.py", line 81, in test_codegen_clean_matcher_classes
[ 118s] self.assertTrue(
[ 118s] AssertionError: False is not true : libcst.matchers.__init__ needs new codegen!
[ 118s]
[ 118s] ======================================================================
[ 118s] FAIL: test_codegen_clean_return_types (libcst.codegen.tests.test_codegen_clean.TestCodegenClean)
[ 118s] Verifies that codegen of return types would not result in a
[ 118s] ----------------------------------------------------------------------
[ 118s] Traceback (most recent call last):
[ 118s] File "/home/abuild/rpmbuild/BUILD/LibCST-0.3.7/libcst/codegen/tests/test_codegen_clean.py", line 116, in test_codegen_clean_return_types
[ 118s] self.assertTrue(
[ 118s] AssertionError: False is not true : libcst.matchers._return_types needs new codegen!
[ 118s]
[ 118s] ======================================================================
[ 118s] FAIL: test_codegen_clean_visitor_functions (libcst.codegen.tests.test_codegen_clean.TestCodegenClean)
[ 118s] Verifies that codegen of visitor functions would not result in a
[ 118s] ----------------------------------------------------------------------
[ 118s] Traceback (most recent call last):
[ 118s] File "/home/abuild/rpmbuild/BUILD/LibCST-0.3.7/libcst/codegen/tests/test_codegen_clean.py", line 47, in test_codegen_clean_visitor_functions
[ 118s] self.assertTrue(
[ 118s] AssertionError: False is not true : libcst._typed_visitor needs new codegen!
[ 118s]
[ 118s] ----------------------------------------------------------------------
[ 118s] Ran 1726 tests in 89.909s
```