During interpreter shutdown, depending on the order in which things happen, a module can be unloaded before all instances of classes defined in that module are garbage collected.
In particular, this means that any global variables (including imported modules) become `None` by the time the instances `__del__` is called, resulting in
```
AttributeError: 'NoneType' object has no attribute 'wc_FreeRng'
```
being displayed while the process exits. This can be avoided simply by catching and ignoring the `AttributeError` in this case, since the process is shutting down anyways.
Python's bytecode compiler has a peephole optimizer which, among other things, can recognize constant expressions and replace them with a constant.
In `Random.byte` the expression `t2b('\0')` is recognized as a constant and is replaced with a single constant compiled into the function's bytecode.
This means that every time you run `Random.byte`, rather than creating a new `str` object (or `bytes` in Python 3) it's reusing the same one each time, and `wc_RNG_GenerateByte` is writing right into that constant object's buffer; hence the following behavior:
```
In [55]: rng = Random()
In [56]: a = rng.byte()
In [57]: a
Out[57]: "'"
In [58]: rng.byte()
Out[58]: '\x11'
In [59]: a
Out[59]: '\x11'
In [60]: rng.byte()
Out[60]: '\x16'
In [61]: a
Out[61]: '\x16'
In [62]: rng.byte.__func__.__code__.co_consts
Out[62]:
('\n Generate and return a random byte.\n ',
'\x16',
0,
'RNG generate byte error (%d)')
In [63]: rng.byte()
Out[63]: '\xad'
In [64]: rng.byte.__func__.__code__.co_consts
Out[64]:
('\n Generate and return a random byte.\n ',
'\xad',
0,
'RNG generate byte error (%d)')
```
`Random.bytes` does not necessarily have this problem since its result buffer is not a constant expression, though I feel like it could also in principle be affected if the string were interned (though I couldn't produce such a result). Nevertheless, it doesn't seem like a good idea to be updating `str` objects' buffers directly.
1. Removed the execute bit from a few C source files.
2. Changed a couple letters in Moises's name in tfm.h to
the non-extended/accented versions of "e" and "a".
1. Opt-out the wolfmath code if not using big integers.
2. Opt-in a few functions when using lean PSK and DTLS.
3. Add a couple (void)heap to hush the compiler for
usused variables in lean PSK.
4. Add include limits.h to internal.h if CHAR_BIT isn't
defined. This is mainly for DTLS with lean PSK.
