chore(tracer): remove support for Python 2 int [backport 2.21] (#13369)

Backport b452b63 from #12693 to 2.21.

We remove support for the Python 2 int type in the encoder.

## Checklist
- [x] PR author has checked that all the criteria below are met
- The PR description includes an overview of the change
- The PR description articulates the motivation for the change
- The change includes tests OR the PR description describes a testing
strategy
- The PR description notes risks associated with the change, if any
- Newly-added code is easy to change
- The change follows the [library release note
guidelines](https://ddtrace.readthedocs.io/en/stable/releasenotes.html)
- The change includes or references documentation updates if necessary
- Backport labels are set (if
[applicable](https://ddtrace.readthedocs.io/en/latest/contributing.html#backporting))

## Reviewer Checklist
- [x] Reviewer has checked that all the criteria below are met 
- Title is accurate
- All changes are related to the pull request's stated goal
- Avoids breaking
[API](https://ddtrace.readthedocs.io/en/stable/versioning.html#interfaces)
changes
- Testing strategy adequately addresses listed risks
- Newly-added code is easy to change
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implications of this PR as reported in the benchmarks PR comment
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[release branch maintenance
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Co-authored-by: Gabriele N. Tornetta <[email protected]>

Author: github-actions[bot]

ddtrace/internal/_encoding.pyx

@@ -107,18 +107,13 @@ cdef inline int pack_number(msgpack_packer *pk, object n) except? -1:
         return msgpack_pack_nil(pk)
 
     if PyLong_Check(n):
-        # PyInt_Check(long) is True for Python 3.
-        # So we should test long before int.
         try:
             if n > 0:
                 return msgpack_pack_unsigned_long_long(pk, <unsigned long long> n)
             return msgpack_pack_long_long(pk, <long long> n)
         except OverflowError as oe:
             raise OverflowError("Integer value out of range")
 
-    if PyInt_Check(n):
-        return msgpack_pack_long(pk, <long> n)
-
     if PyFloat_Check(n):
         return msgpack_pack_double(pk, <double> n)
 
@@ -1024,8 +1019,6 @@ cdef class Packer(object):
             if o is None:
                 ret = msgpack_pack_nil(&self.pk)
             elif PyLong_CheckExact(o):
-                # PyInt_Check(long) is True for Python 3.
-                # So we should test long before int.
                 try:
                     if o > 0:
                         ullval = o
@@ -1040,9 +1033,6 @@ cdef class Packer(object):
                         continue
                     else:
                         raise OverflowError("Integer value out of range")
-            elif PyInt_CheckExact(o):
-                longval = o
-                ret = msgpack_pack_long(&self.pk, longval)
             elif PyFloat_CheckExact(o):
                 dval = o
                 ret = msgpack_pack_double(&self.pk, dval)