Most of the import sequence now uses per-module locks rather than the
global import lock, eliminating well-known issues with threads and imports.

directory.

Issue #14780: urllib.request.urlopen() now has a `cadefault` argument to use the default certificate store.
Initial patch by James Oakley.

```--------------------
  1) Reduce the number of iterations in the Horner scheme for operands with
     a negative adjusted exponent. Previously the number was overestimated
     quite generously.

  2) The function _mpd_get_exp_iterations() now has an ACL2 proof and
     is rewritten accordingly.

  3) The proof relies on abs(op) > 9 * 10**(-prec-1), so operands without
     that property are now handled by the new function _mpd_qexp_check_one().

  4) The error analysis for the evaluation of the truncated Taylor series
     in Hull&Abrham's paper relies on the fact that the reduced operand
     'r' has fewer than context.prec digits.

     Since the operands may have more than context.prec digits, a new ACL2
     proof covers the case that r.digits > context.prec. To facilitate the
     proof, the Horner step now uses fma instead of rounding twice in
     multiply/add.


Changes in mpd_qexp():
```

-------------------

  1) Fix a bound in the correct rounding loop that was too optimistic. In
     practice results were always correctly rounded, because it is unlikely
     that the error in _mpd_qexp() ever reaches the theoretical maximum.

The behaviour of the parse_constant callback changed insofar that 'null',
'true', 'false' don't trigger its call anymore.

Patch by Serhiy Storchaka

The behaviour of the parse_constant callback changed insofar that 'null',
'true', 'false' don't trigger its call anymore.

Patch by Serhiy Storchaka

Issue #14693: Under non-Windows platforms, hashlib's fallback modules are always compiled, even if OpenSSL is present at build time.

(untested, because of Windows build issues under 3.x)

Patch by Robin Schreiber.

Patch by EungJun Yi.

undecoded UTF-8 bytes as a string when accessing the Tk clipboard.
Modify clipboad_get() to first request type UTF8_STRING when no
specific type is requested in an X11 windowing environment, falling
back to the current default type STRING if that fails.
Original patch by Thomas Kluyver.

Patch by Serhiy Storchaka.