Python: Fix bad join in MethodCallsiteRefinement

Observed on `FreeCAD/FreeCAD`:

```
Tuple counts for Essa::MethodCallsiteRefinement#24e22a14#f/1@274967ic after 34.5s:
638284     ~0%     {2} r1 = SCAN Essa::TEssaNodeRefinement#24e22a14#ffff OUTPUT In.0, In.3 'this'
636521     ~0%     {2} r2 = r1 AND NOT Essa::SingleSuccessorGuard#class#24e22a14#f(Lhs.1 'this')
1579493668 ~0%     {2} r3 = JOIN r2 WITH SsaDefinitions::SsaSource::method_call_refinement#9197156e#fff ON FIRST 1 OUTPUT Lhs.1 'this', Rhs.2
266673     ~3%     {1} r4 = JOIN r3 WITH Essa::EssaNodeRefinement::getDefiningNode#dispred#f0820431#ff ON FIRST 2 OUTPUT Lhs.0 'this'
                    return r4
```

After a bit of unbinding, we have:

```
Tuple counts for Essa::MethodCallsiteRefinement#24e22a14#f/1@d73d8e27 after 66ms:
215168 ~1%     {2} r1 = SCAN Definitions::SsaSourceVariable#class#486534ab#f OUTPUT In.0, In.0
283965 ~2%     {2} r2 = JOIN r1 WITH SsaDefinitions::SsaSource::method_call_refinement#9197156e#fff ON FIRST 1 OUTPUT Rhs.2, Lhs.1
401274 ~0%     {2} r3 = JOIN r2 WITH Essa::EssaNodeRefinement::getDefiningNode#dispred#f0820431#ff_10#join_rhs ON FIRST 1 OUTPUT Lhs.1, Rhs.1 'this'
266671 ~2%     {1} r4 = JOIN r3 WITH Essa::TEssaNodeRefinement#24e22a14#ffff_03#join_rhs ON FIRST 2 OUTPUT Lhs.1 'this'
266671 ~2%     {1} r5 = r4 AND NOT Essa::SingleSuccessorGuard#class#24e22a14#f(Lhs.0 'this')
                return r5
```
(I'm somewhat confused about the slight difference in tuples, but it's
probably just because the compiler moved some stuff around.)

Author: tausbn

python/ql/lib/semmle/python/essa/Essa.qll

@@ -764,7 +764,8 @@ class CallsiteRefinement extends EssaNodeRefinement {
 /** An implicit (possible) modification of the object referred at a method call */
 class MethodCallsiteRefinement extends EssaNodeRefinement {
   MethodCallsiteRefinement() {
-    SsaSource::method_call_refinement(this.getSourceVariable(), _, this.getDefiningNode()) and
+    SsaSource::method_call_refinement(pragma[only_bind_into](this.getSourceVariable()), _,
+      this.getDefiningNode()) and
     not this instanceof SingleSuccessorGuard
   }