Specify minimum requirements for dependencies (#13317)

Specifies minimum allowed versions of `colorama`, `iniconfig`, and `packaging`, and bumps the minimum requirement of `exceptiongroup` to no longer be a release candidate.  

This specifies minimum allowed versions that are several years old, but these can be changed to be more recent.

## Motivation

[`uv`](https://docs.astral.sh/uv/) has multiple dependency [resolution strategies](https://docs.astral.sh/uv/concepts/resolution/#resolution-strategy):

 - `lowest` will install the lowest allowed version for _all_ dependencies, both direct and indirect (transitive).
 - `lowest-direct` will use the lowest compatible versions for all _direct_ dependencies, while using the latest compatible versions for all other dependencies.

Up until now, `pytest` has not specified the minimum allowed versions of multiple dependencies in `pyproject.toml`.  When I ran `uv pip install pytest==8.3.5 --resolution=lowest` in a fresh virtual environment, it installed `packaging==14.0` (released in 2014 👀) and `iniconfig==0.1` (released in 2010 😅).

It's a fairly common practice in the scientific pythoniverse to run tests against the oldest versions of dependencies.  In practice, using `uv`'s `lowest` dependency resolution strategy results the first release of certain indirect dependencies being installed, like `v0.0.0.1`.  Specifying lower bounds will make it a lot more practical for `pytest` users to test against the oldest allowed versions of both indirect and direct dependencies.

Author: namurphy

changelog/13317.packaging.rst

@@ -0,0 +1,4 @@
+Specified minimum allowed versions of ``colorama``, ``iniconfig``,
+and ``packaging``; and bumped the minimum allowed version
+of ``exceptiongroup`` for ``python_version<'3.11'`` from a release
+candidate to a full release.

pyproject.toml

@@ -46,10 +46,10 @@ dynamic = [
     "version",
 ]
 dependencies = [
-    "colorama; sys_platform=='win32'",
-    "exceptiongroup>=1.0.0rc8; python_version<'3.11'",
-    "iniconfig",
-    "packaging",
+    "colorama>=0.4; sys_platform=='win32'",
+    "exceptiongroup>=1; python_version<'3.11'",
+    "iniconfig>=1",
+    "packaging>=20",
     "pluggy>=1.5,<2",
     "pygments>=2.7.2",
     "tomli>=1; python_version<'3.11'",