Python: Use Python 3 for investigation tests

Apparently there are minor differences with `test-6-max-import-depth-2`
where under Python 2 `isfile_no_problem.py` still works as before

Author: RasmusWL

python/ql/test/experimental/dataflow/strange-pointsto-interaction-investigation/README.md

@@ -81,7 +81,7 @@ $ diff -u -r test-4-max-import-depth-100/ test-5-max-import-depth-3/
 +semmle-extractor-options: --max-import-depth=3 -R ../src
 ```
 
-Also notice that using import depth 2 actually makes things worse, as we no longer handle the `isfile_no_problem.py` file properly :facepalm: :sweat_smile:
+Also notice that using import depth 2 actually makes things worse, as we no longer handle the `isfile_no_problem.py` file properly :facepalm: :sweat_smile: NOTE: This was only for Python 3, for Python 2 there was no change :flushed:
 
 ```diff
 diff '--color=auto' -u -r test-4-max-import-depth-100/NormalDataflowTest.expected test-6-max-import-depth-2/NormalDataflowTest.expected

python/ql/test/experimental/dataflow/strange-pointsto-interaction-investigation/test-1-normal/options

@@ -1 +1 @@
-semmle-extractor-options: --max-import-depth=1 -R ../src
+semmle-extractor-options: --lang=3 --max-import-depth=1 -R ../src

python/ql/test/experimental/dataflow/strange-pointsto-interaction-investigation/test-2-without-splitting/options

@@ -1 +1 @@
-semmle-extractor-options: --dont-split-graph --max-import-depth=1 -R ../src
+semmle-extractor-options: --lang=3 --dont-split-graph --max-import-depth=1 -R ../src

python/ql/test/experimental/dataflow/strange-pointsto-interaction-investigation/test-3-max-import-depth-0/options

@@ -1 +1 @@
-semmle-extractor-options: --max-import-depth=0 -R ../src
+semmle-extractor-options: --lang=3 --max-import-depth=0 -R ../src

python/ql/test/experimental/dataflow/strange-pointsto-interaction-investigation/test-4-max-import-depth-100/options

@@ -1 +1 @@
-semmle-extractor-options: --max-import-depth=100 -R ../src
+semmle-extractor-options: --lang=3 --max-import-depth=100 -R ../src

python/ql/test/experimental/dataflow/strange-pointsto-interaction-investigation/test-5-max-import-depth-3/options

@@ -1 +1 @@
-semmle-extractor-options: --max-import-depth=3 -R ../src
+semmle-extractor-options: --lang=3 --max-import-depth=3 -R ../src

python/ql/test/experimental/dataflow/strange-pointsto-interaction-investigation/test-6-max-import-depth-2/options

@@ -1 +1 @@
-semmle-extractor-options: --max-import-depth=2 -R ../src
+semmle-extractor-options: --lang=3 --max-import-depth=2 -R ../src