pypy support: fix a cpyext incompatibility in chunkers/reader, adjust test expectations, see #1755

[ThomasWaldmann]

Summary

With this PR, borg works on pypy3.11, tested with a py3.11 nightly build
(pypy-c-jit-187982-8ed7cc691813, PyPy 8.0.0-alpha0 / Python 3.11.15) on macOS/arm64:
the full test suite passes there (2534 passed, 384 skipped, 1 xfailed), and CPython is
unaffected (full suite verified, no regressions). A nightly is required - the pypy fixes borg
needs are not in a release yet.

Changes

chunkers/reader: memcpy instead of memoryview slice assignment. This is what currently
keeps borg from running on pypy at all: mv[a:b] = src raises TypeError when the statement
is compiled into a C extension - it works in interpreted code, and item assignment works -
because pypy's cpyext memoryview lacks the assignment slot (pypy/pypy#5564). Without this,
506 of ~2900 tests fail, i.e. everything that writes an archive. Copying via memcpy()
through Cython typed memoryviews avoids creating the slice objects and is a wash on CPython:
1.689 vs 1.680 GB/s (best of 6, chunking a 2 GiB file through the block reader path).

chunkers/reader: limit the read size in the no-readv fallback. Not pypy-specific, but
that is where it hurts most: without os.readv (win32, pypy - pypy/pypy#5565) we read with
os.read, which allocates a buffer of the requested size no matter how few bytes it then
returns (pypy also zeroes it, pypy/pypy#5566). Asking for the whole free scan buffer (up to
8 MiB) therefore costs MBs of allocation per small file. Backing up 20000 small files
(82.6 MiB of data) requested 468 GiB in 60002 read calls; capping the request at 256 KiB -
same call count, same data - cuts the time in os.read from 15.1 s to 1.3 s:

	uncapped	capped at 256 KiB
pypy, 20000 small files	27.6 s	12.3 s
CPython without readv (= win32), same	4.04 s	2.42 s (readv path: 2.36 s)
chunking a 2 GiB file	unaffected	unaffected

pypy support proper, all of it about behaviour that is not a pypy bug:

• platformflags: add is_pypy.
• item_test: xfail test_unknown_property - setting undeclared attributes on cdef class
  instances is not blocked under cpyext.
• msgpack_test: pypy only has the pure-python msgpack, expect it to be slow there.
• lock_cmds_test: tolerate the pure-python msgpack warning on stderr, and use
  sys.executable instead of "python3" (robustness, not pypy-specific).

Performance

pypy remains slower than CPython for borg's workload and this PR does not change that:
borg create of a 2 GiB file is 4.8 s vs 10.8 s, 20000 small files 2.4 s vs 12.3 s (it was
27 s before the read-size cap above). Profiling says the rest is the cpyext tax, spread over
all of borg's C extensions rather than one hot spot - a trivial C ext call is 304 ns on pypy
vs 45 ns on CPython (a pure python call is 1 ns vs 50 ns), a 4 KiB bytes object created in
C 3.0 us vs 0.6 us, AES-OCB on a 2 KiB chunk 20.8 us vs 2.0 us, lz4 12.2 us vs 1.0 us,
ChunkIndex set+get 20.2 us vs 1.8 us. Where borg is plain Python, pypy is faster
(borg create -n, walk and stat only: 0.71 s vs 0.43 s including interpreter startup).
So: compatibility, not speed.

See #1755. The CI job running the suite on a pypy nightly came from #10142 and #10143.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.88%. Comparing base (7d81c68) to head (4be2170).
⚠️ Report is 1 commits behind head on master.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #9976      +/-   ##
==========================================
- Coverage   86.90%   86.88%   -0.02%     
==========================================
  Files         101      101              
  Lines       17863    17864       +1     
  Branches     2708     2708              
==========================================
- Hits        15523    15521       -2     
- Misses       1636     1637       +1     
- Partials      704      706       +2     

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