[3.7] Add missed details of the C API introduced in 3.7. (GH-7047) (GH-7061)

* Set the limited API version for PyImport_GetModule and PyOS_*Fork
  functions.
* Add PyImport_GetModule and Py_UTF8Mode in PC/python3.def.
* Add several functions in Doc/data/refcounts.dat.
(cherry picked from commit 4e29f566e8821c09d8274eadcdd355e8b1284b8b)
diff --git a/PC/python3.def b/PC/python3.def
index a4c0415..9e0348b 100644
--- a/PC/python3.def
+++ b/PC/python3.def
@@ -284,6 +284,7 @@
   PyImport_GetImporter=python37.PyImport_GetImporter
   PyImport_GetMagicNumber=python37.PyImport_GetMagicNumber
   PyImport_GetMagicTag=python37.PyImport_GetMagicTag
+  PyImport_GetModule=python37.PyImport_GetModule
   PyImport_GetModuleDict=python37.PyImport_GetModuleDict
   PyImport_Import=python37.PyImport_Import
   PyImport_ImportFrozenModule=python37.PyImport_ImportFrozenModule
@@ -757,6 +758,7 @@
   Py_SetPythonHome=python37.Py_SetPythonHome
   Py_SetRecursionLimit=python37.Py_SetRecursionLimit
   Py_SymtableString=python37.Py_SymtableString
+  Py_UTF8Mode=python37.Py_UTF8Mode DATA
   Py_VaBuildValue=python37.Py_VaBuildValue
   _PyArg_ParseTupleAndKeywords_SizeT=python37._PyArg_ParseTupleAndKeywords_SizeT
   _PyArg_ParseTuple_SizeT=python37._PyArg_ParseTuple_SizeT