modstruct: Improve compliance with python3

While checking whether we can enable -Wimplicit-fallthrough, I encountered
a diagnostic in mp_binary_set_val_array_from_int which led to discovering
the following bug:
```
>>> struct.pack("xb", 3)
b'\x03\x03'
```
That is, the next value (3) was used as the value of a padding byte, while
standard Python always fills "x" bytes with zeros.  I initially thought
this had to do with the unintentional fallthrough, but it doesn't.
Instead, this code would relate to an array.array with a typecode of
padding ('x'), which is ALSO not desktop Python compliant:
```
>>> array.array('x', (1, 2, 3))
array('x', [1, 0, 0])
```
Possibly this is dead code that used to be shared between struct-setting
and array-setting, but it no longer is.

I also discovered that the argument list length for struct.pack
and struct.pack_into were not checked, and that the length of binary data
passed to array.array was not checked to be a multiple of the element
size.

I have corrected all of these to conform more closely to standard Python
and revised some tests where necessary.  Some tests for micropython-specific
behavior that does not conform to standard Python and is not present
in CircuitPython was deleted outright.

1 files changed, 0 insertions, 41 deletions

diff --git a/tests/basics/struct_micropython.py b/tests/basics/struct_micropython.py
deleted file mode 100644
index e72153071..000000000
--- a/tests/basics/struct_micropython.py
+++ /dev/null
@@ -1,41 +0,0 @@
-# test MicroPython-specific features of struct
-
-try:
-    import ustruct as struct
-except:
-    try:
-        import struct
-    except ImportError:
-        print("SKIP")
-        raise SystemExit
-
-try:
-    struct.pack('O', None)
-except ValueError:
-    print("SKIP")
-    raise SystemExit
-
-class A():
-    pass
-
-# pack and unpack objects
-o = A()
-s = struct.pack("<O", o)
-o2 = struct.unpack("<O", s)
-print(o is o2[0])
-
-# pack can accept less arguments than required for the format spec
-print(struct.pack('<2I', 1))
-
-# pack and unpack pointer to a string
-# This requires uctypes to get the address of the string and instead of
-# putting this in a dedicated test that can be skipped we simply pass
-# if the import fails.
-try:
-    import uctypes
-    o = uctypes.addressof('abc')
-    s = struct.pack("<S", o)
-    o2 = struct.unpack("<S", s)
-    assert o2[0] == 'abc'
-except ImportError:
-    pass