tests: Make "io" modules fixes for CPython compatibility.

Previously, "import _io" worked on both CPython and MicroPython (essentially
by a chance on CPython, as there's not guarantee that its contents will stay
the same across versions), but as the module was renamed to uio, need to use
more robust import sequence for compatibility.

tests/io/stringio1.py

@@ -1,4 +1,7 @@
-import uio as io
+try:
+    import uio as io
+except ImportError:
+    import io
 
 a = io.StringIO()
 print('io.StringIO' in repr(a))

tests/io/stringio_with.py

@@ -1,4 +1,7 @@
-import uio as io
+try:
+    import uio as io
+except ImportError:
+    import io
 
 # test __enter__/__exit__
 with io.StringIO() as b:

tests/misc/print_exception.py

@@ -1,4 +1,7 @@
-import uio as io # uPy does not have io module builtin
+try:
+    import uio as io
+except ImportError:
+    import io
 import sys
 if hasattr(sys, 'print_exception'):
     print_exception = sys.print_exception