Fixes https://github.com/cython/cython/issues/2093

See https://github.com/cython/cython/pull/3677

Disable Py_UNICODE fallback for __Pyx_UnicodeContainsUCS4() in Py3.9 since Py_UNICODE is deprecated and PEP-393 unicode is practically required.

* Validate and fix temp releasing (GH-3708)
Backports 92147baf.

    * Fix a temp leak in the type init code.
    * Fix temp leaks in fused types initialisation code.
    * Correctly release the buffer index temps allocated for index calculations.
    * Make tests fails hard if a temp variable is not released at the end of a generated function.
    * Fix temp leak in switch statement code.
    * Make end-to-end tests fail on refnanny output.
    * Fix result temp leak in PyTypeTestNode.
    * Fix result temp leak in external type/function import code and enable the refnanny check for them.
    * Fix temp leak when try-return-finally is used in generators.
    * Make it explicit when an allocated temp is not meant to be reused.
    * Fix temp leak when assigning to the real/imag attributes of complex numbers.
    * Fix temp leak when assigning to a memoryview slice.
    * Clean up "num_threads" result temp in parallel section, not only in prange loop.
    * Fix temp leak in Pythran buffer setitem code.
    * Simplify NumPyMethodCallNode since it does not need the Python function anymore. Previously, it generated code that needlessly looked up the Python function without actually using it.
    * Fix temp leak when deleting C++ objects.
    * Add a test that actually reusing temps when deleting C++ objects works correctly.

* Really only use PyUnicode_FromUnicode() for strings that contain lone surrogate, not for normal non-BMP strings and not for surrogate pairs on 16bit Unicode platforms.

See https://github.com/cython/cython/issues/3678

* Extend buildenv test to debug a MacOS problem.
* Add a test for surrogate pairs in Unicode strings.
* Limit PyUnicode_FromUnicode() usage to strings containing lone surrogates.
* Accept ambiguity of surrogate pairs in Unicode string literals when generated on 16bit Py2 systems.

Implement generic optimized loop iterator with indexing and type inference for memoryviews (GH-3617)

* Adds bytearray iteration since that was not previously optimised (because it allows changing length during iteration).
* Always set `entry.init` for memoryviewslice.

Fixes #3712 (hopefully) by reverting a small part of bbef4d74

Fixes #3712 (hopefully) by reverting a small part of bbef4d74

* Fix a temp leak in the type init code.
* Fix temp leaks in fused types initialisation code.
* Correctly release the buffer index temps allocated for index calculations.
* Make tests fails hard if a temp variable is not released at the end of a generated function.
* Fix temp leak in switch statement code.
* Make end-to-end tests fail on refnanny output.
* Fix result temp leak in PyTypeTestNode.
* Fix result temp leak in external type/function import code and enable the refnanny check for them.
* Fix temp leak when try-return-finally is used in generators.
* Make it explicit when an allocated temp is not meant to be reused.
* Fix temp leak when assigning to the real/imag attributes of complex numbers.
* Fix temp leak when assigning to a memoryview slice.
* Clean up "num_threads" result temp in parallel section, not only in prange loop.
* Fix temp leak in Pythran buffer setitem code.
* Simplify NumPyMethodCallNode since it does not need the Python function anymore. Previously, it generated code that needlessly looked up the Python function without actually using it.
* Fix temp leak when deleting C++ objects.
* Add a test that actually reusing temps when deleting C++ objects works correctly.

`PyModule_GetDict` is part of the limited API so we can keep a reference
to the module dict around regardless of limited mode being enabled or not.

Avoid a call to PyTuple_GET_ITEM() to get the item array pointer if CYTHON_ASSUME_SAFE_MACROS is disabled.
See https://github.com/cython/cython/issues/3701

exec() did not allow recent Python syntax features in Py3.8+ due to https://bugs.python.org/issue35975
Closes https://github.com/cython/cython/issues/3695