Misc fixes

[skirpichev]

• use pythoncapi-compat's pythoncapi_compat.h
• use _PyLong_Sign() instead of _PyLong_IsNegative()
• removed build-wheels.conf // seems to be unused
• reuse mpz_set_PyLong in GMPy_MPZ_From_PyLong() and GMPy_XMPZ_From_PyLong()
• fix install.rst (move up "From Sources" section)
• move changelogs for 1.x versions from gmpy2.c to history.rst
• move changelogs for 2.0.0b1 from gmpy2.c to history.rst
• move changelogs for 2.0.0b[234] from gmpy2.c to history.rst
• move changelogs for 2.0.0 and 2.1.x from gmpy2.c to history.rst
• update changelog for 2.2.0 (from gmpy2.c), closes https://gmpy2.readthedocs.io/en/latest/history.html outdated #329

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 89.47368% with 4 lines in your changes are missing coverage. Please review.

Project coverage is 85.52%. Comparing base (eb8dfcb) to head (283c665).
Report is 7 commits behind head on master.

Files	Patch %	Lines
src/gmpy2_cache.c	50.00%	4 Missing ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #471      +/-   ##
==========================================
- Coverage   85.55%   85.52%   -0.03%     
==========================================
  Files          49       50       +1     
  Lines       11678    11656      -22     
  Branches     2209     2202       -7     
==========================================
- Hits         9991     9969      -22     
  Misses       1687     1687              

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[casevh]

Looks good. Thanks for all the cleanup.

casevh