fix: pin ddtrace to <4.8.0 to avoid import errors

[brettlangdon]

What does this PR do?

Pin ddtrace dependency to <4.8.0

Motivation

datadog-lambda-python was relying on an internal/private module from dd-trace-py which has changed in 4.8.0. We did not catch the incompatibility until after 4.8.0 has been released.

Example error:

ImportError while importing test module '/go/src/github.com/DataDog/datadog-lambda-python/tests/test_tracing.py'.
Hint: make sure your test modules/packages have valid Python names.
Traceback:
venv/lib/python3.11/site-packages/_pytest/python.py:507: in importtestmodule
    mod = import_path(
venv/lib/python3.11/site-packages/_pytest/pathlib.py:587: in import_path
    importlib.import_module(module_name)
/usr/local/lib/python3.11/importlib/__init__.py:126: in import_module
    return _bootstrap._gcd_import(name[level:], package, level)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
<frozen importlib._bootstrap>:1204: in _gcd_import
    ???
<frozen importlib._bootstrap>:1176: in _find_and_load
    ???
<frozen importlib._bootstrap>:1147: in _find_and_load_unlocked
    ???
<frozen importlib._bootstrap>:690: in _load_unlocked
    ???
datadog_lambda/cold_start.py:130: in wrapped_method
    return original_exec_module(module)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
datadog_lambda/cold_start.py:130: in wrapped_method
    return original_exec_module(module)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
venv/lib/python3.11/site-packages/_pytest/assertion/rewrite.py:197: in exec_module
    exec(co, module.__dict__)
tests/test_tracing.py:15: in <module>
    from ddtrace._trace._span_pointer import _SpanPointer
E   ImportError: cannot import name '_SpanPointer' from 'ddtrace._trace._span_pointer' (/go/src/github.com/DataDog/datadog-lambda-python/venv/lib/python3.11/site-packages/ddtrace/_trace/_span_pointer.py)

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Additional Notes

Types of Changes

• Bug fix
• New feature
• Breaking change
• Misc (docs, refactoring, dependency upgrade, etc.)

Check all that apply

• This PR's description is comprehensive
• This PR contains breaking changes that are documented in the description
• This PR introduces new APIs or parameters that are documented and unlikely to change in the foreseeable future
• This PR impacts documentation, and it has been updated (or a ticket has been logged)
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