Enable PY_SSIZE_T_CLEAN in order to avoid deprecation warning and/or segfault

under Python 3.8.0b1 (https://github.com/oracle/python-cx_Oracle/issues/317).

src/cxoConnection.c

@@ -1199,7 +1199,7 @@ static PyObject *cxoConnection_commit(cxoConnection *conn, PyObject *args)
 //-----------------------------------------------------------------------------
 static PyObject *cxoConnection_begin(cxoConnection *conn, PyObject *args)
 {
-    uint32_t transactionIdLength, branchIdLength;
+    Py_ssize_t transactionIdLength, branchIdLength;
     const char *transactionId, *branchId;
     int formatId, status;
 

src/cxoModule.h

@@ -9,6 +9,8 @@
 //   Include file for all cx_Oracle source files.
 //-----------------------------------------------------------------------------
 
+#define PY_SSIZE_T_CLEAN                1
+
 #include <Python.h>
 #include <structmember.h>
 #include <time.h>

src/cxoSessionPool.c

@@ -332,7 +332,7 @@ static PyObject *cxoSessionPool_acquire(cxoSessionPool *pool, PyObject *args,
             "tag", "matchanytag", "shardingkey", "supershardingkey", NULL };
     PyObject *createKeywordArgs, *result, *cclassObj, *purityObj, *tagObj;
     PyObject *shardingKeyObj, *superShardingKeyObj;
-    unsigned usernameLength, passwordLength;
+    Py_ssize_t usernameLength, passwordLength;
     char *username, *password;
     PyObject *matchAnyTagObj;