Closes #14842.

Closes #14842.

that it can be imported when threads are disabled.
(followup to issue #12098)

Initial patch by Sergey Mezentsev.

Patch by Justin Peel.

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