apacheGH-38984: [Python][Packaging] Verification of wheels on AlmaLin…

…ux 8 are failing due to missing pip (apache#38985)

### Rationale for this change

Almalinux 8 has been updated from 8.8 to 8.9. When using 8.9 python3 seems to be shipped without pip as  the command `python3 -m pip install -U pip` fails to find pip.

### What changes are included in this PR?

Use the [ensurepip package](https://docs.python.org/3/library/ensurepip.html) which provides support for bootstrapping the pip installer into an existing Python installation.

### Are these changes tested?

Yes via archery.

### Are there any user-facing changes?

No
* Closes: apache#38984

Authored-by: Raúl Cumplido <raulcumplido@gmail.com>
Signed-off-by: Sutou Kouhei <kou@clear-code.com>

dev/release/setup-rhel-rebuilds.sh

@@ -44,8 +44,6 @@ dnf -y install \
   ninja-build \
   nodejs \
   openssl-devel \
-  python38-devel \
-  python38-pip \
   ruby-devel \
   sqlite-devel \
   vala-devel \
@@ -54,5 +52,5 @@ dnf -y install \
 
 npm install -g yarn
 
-python3 -m pip install -U pip
+python3 -m ensurepip --upgrade
 alternatives --set python /usr/bin/python3