v3.1.10 commits and release notes

[cary-ilm]

Commits for a v3.1.10 patch release. No ABI changes, just build/bug/test fixes.

Build fixes:

• Fix a problem where downstream projects using Imath would build python bindings even if they weren't requested.
• Fix for missing std::bit_cast
• Fix missing/necessary use of IMATH_HOSTDEVICE
• IMATH_INSTALL_PKG_CONFIG is now on by default, even on Windows
• Fix calling default constructor by uniform init in TypeTraits
• Fix redundant PYIMATH_EXPORTS causing compile issues on Windows Clang

• Update to SO versioning policy:

  This change adopts a policy of appending the MAJOR.MINOR.PATCH software release name to the SONAME to form the real name of the shared library.

  See website/install.rst and #339 for more details.

Bug fixes:

• Fix half to float giving wrong results on older x86_64 CPUs on Windows

Tests:

• Expand epsilon bounds for m44x pyImath test.

Other:

• succ()/pred() now use std::nextafter().
• Rename "docs" to "website".
• Add missing copyright/license identifiers.