tests/basics: bytes/str.partition/rpartition are now optional.

Skip tests if not available.

tests/basics/bytes_partition.py

+try:
+    str.partition
+except AttributeError:
+    print("SKIP")
+    import sys
+    sys.exit()
+
 print(b"asdf".partition(b'g'))
 print(b"asdf".partition(b'a'))
 print(b"asdf".partition(b's'))

tests/basics/string_partition.py

+try:
+    str.partition
+except AttributeError:
+    print("SKIP")
+    import sys
+    sys.exit()
+
 print("asdf".partition('g'))
 print("asdf".partition('a'))
 print("asdf".partition('s'))

tests/basics/string_rpartition.py

+try:
+    str.partition
+except AttributeError:
+    print("SKIP")
+    import sys
+    sys.exit()
+
 print("asdf".rpartition('g'))
 print("asdf".rpartition('a'))
 print("asdf".rpartition('s'))