Reduce the size of the build by not compiling parts of stubgen (#10107)

This might fix the out-of-memory errors we are getting when
building Windows wheels. In any case these will speed up
builds slightly.

The `stubgen.py` file must be compiled since it uses inheritance
from native classes. This removes one case of inheritance, but
the others are harder to get rid of.

Author: JukkaL

mypy/stubgen.py

@@ -192,18 +192,26 @@ def __init__(self,
         self.export_less = export_less
 
 
-class StubSource(BuildSource):
+class StubSource:
     """A single source for stub: can be a Python or C module.
 
     A simple extension of BuildSource that also carries the AST and
     the value of __all__ detected at runtime.
     """
     def __init__(self, module: str, path: Optional[str] = None,
                  runtime_all: Optional[List[str]] = None) -> None:
-        super().__init__(path, module, None)
+        self.source = BuildSource(path, module, None)
         self.runtime_all = runtime_all
         self.ast = None  # type: Optional[MypyFile]
 
+    @property
+    def module(self) -> str:
+        return self.source.module
+
+    @property
+    def path(self) -> Optional[str]:
+        return self.source.path
+
 
 # What was generated previously in the stub file. We keep track of these to generate
 # nicely formatted output (add empty line between non-empty classes, for example).
@@ -1435,7 +1443,7 @@ def generate_asts_for_modules(py_modules: List[StubSource],
         return
     # Perform full semantic analysis of the source set.
     try:
-        res = build(list(py_modules), mypy_options)
+        res = build([module.source for module in py_modules], mypy_options)
     except CompileError as e:
         raise SystemExit("Critical error during semantic analysis: {}".format(e)) from e
 

setup.py

@@ -99,8 +99,11 @@ def run(self):
         # Also I think there would be problems with how we generate version.py.
         'version.py',
 
-        # Can be removed once we drop support for Python 3.5.2 and lower.
+        # Skip these to reduce the size of the build
         'stubtest.py',
+        'stubgenc.py',
+        'stubdoc.py',
+        'stubutil.py',
     )) + (
         # Don't want to grab this accidentally
         os.path.join('mypyc', 'lib-rt', 'setup.py'),