This commit adds the errno attribute to exceptions, so code can retrieve
errno codes from an OSError using exc.errno.

The implementation here simply lets `errno` (and the existing `value`)
attributes work on any exception instance (they both alias args[0]).  This
is for efficiency and to keep code size down.  The pros and cons of this
are:

Pros:
- more compatible with CPython, less difference to document and learn
- OSError().errno will correctly return None, whereas the current way of
  doing it via OSError().args[0] will raise an IndexError
- it reduces code size on most bare-metal ports (because they already have
  the errno qstr)
- for Python code that uses exc.errno the generated bytecode is 2 bytes
  smaller and more efficient to execute (compared with exc.args[0]); so
  bytecode loaded to RAM saves 2 bytes RAM for each use of this attribute,
  and bytecode that is frozen saves 2 bytes flash/ROM for each use
- it's easier/shorter to type, and saves 2 bytes of space in .py files that
  use it (for each use)

Cons:
- increases code size by 4-8 bytes on minimal ports that don't already have
  the `errno` qstr
- all exceptions now have .errno and .value attributes (a cpydiff test is
  added to address this)

See also #2407.

Signed-off-by: Damien George <damien@micropython.org>

For certain operands to mpn_div, the existing code path for
`DIG_SIZE == MPZ_DBL_DIG_SIZE / 2` had a bug in it where borrow could still
overflow in the `(x >= *n || *n - x <= borrow)` branch, ie
`borrow + x - (mpz_dbl_dig_t)*n` overflows the borrow variable.  In such
cases the subsequent right-shift of borrow would not bring in the overflow
bit, leading to an error in the result.  An example division that had
overflow when MPZ_DIG_SIZE = 16 is `(2 ** 48 - 1) ** 2 // (2 ** 48 - 1)`.

This is fixed in this commit by simplifying the code and handling the low
digits of borrow first, and then the upper bits (to shift down) separately.
There is no longer a distinction between `DIG_SIZE < MPZ_DBL_DIG_SIZE / 2`
and `DIG_SIZE == MPZ_DBL_DIG_SIZE / 2`.

This commit also simplifies the second part of the calculation so that
borrow does not need to be negated (instead the code just works knowing
that borrow is negative and using + instead of - in calculations involving
borrow).

Fixes #6777.

Signed-off-by: Damien George <damien@micropython.org>

This returns a reference to the globals dict associated with the function,
ie the global scope that the function was defined in.  This attribute is
read-only but the dict itself is modifiable, per CPython behaviour.

Signed-off-by: Damien George <damien@micropython.org>

Signed-off-by: Damien George <damien@micropython.org>

The use of -S ensures that only the CPython standard library is accessible,
which makes tests run the same regardless of any site-packages that are
installed.  It also improves start-up time of CPython, reducing the overall
time spent running the test suite.

tests/basics/containment.py is updated to work around issue with old Python
versions not being able to str-format a dict-keys object, which becomes
apparent when -S is used.

Signed-off-by: Damien George <damien@micropython.org>

And enable this feature on unix, the coverage variant.  The .exp test file
is needed so the test can run on CPython versions prior to "@=" operator
support.

Signed-off-by: Damien George <damien@micropython.org>

These work since 3aab54bf434e7f025a91ea05052f1bac439fad8c

Signed-off-by: Damien George <damien@micropython.org>

This is consistent with the other 'micro' modules and allows implementing
additional features in Python via e.g. micropython-lib's sys.

Note this is a breaking change (not backwards compatible) for ports which
do not enable weak links, as "import sys" must now be replaced with
"import usys".

As per CPython behaviour, compile(stmt, "file", "single") should create
code which prints to stdout (via __repl_print__) the results of any
expressions in stmt.

Signed-off-by: Damien George <damien@micropython.org>