HADOOP-17496. Install a supported version of pip

[GauthamBanasandra]

• pip version 21 ended support for
  python 2.7. Thus we need to install
  a version of pip that supports
  python 2.7.

[GauthamBanasandra]

End of python2.7 support for pip 21 - pypa/pip#9500.

[hadoop-yetus]

(!) A patch to the testing environment has been detected.
Re-executing against the patched versions to perform further tests.
The console is at https://ci-hadoop.apache.org/job/hadoop-multibranch/job/PR-2653/1/console in case of problems.

[hadoop-yetus]

🎊 +1 overall

Vote	Subsystem	Runtime	Logfile	Comment
+0 🆗	reexec	35m 1s		Docker mode activated.
			_ Prechecks _
+1 💚	dupname	0m 0s		No case conflicting files found.
+1 💚	@author	0m 0s		The patch does not contain any @author tags.
			_ trunk Compile Tests _
+0 🆗	mvndep	14m 15s		Maven dependency ordering for branch
+1 💚	shadedclient	19m 58s		branch has no errors when building and testing our client artifacts.
			_ Patch Compile Tests _
+0 🆗	mvndep	0m 31s		Maven dependency ordering for patch
+1 💚	hadolint	0m 5s		There were no new hadolint issues.
+1 💚	shellcheck	0m 0s		There were no new shellcheck issues.
+1 💚	shelldocs	0m 13s		There were no new shelldocs issues.
+1 💚	whitespace	0m 0s		The patch has no whitespace issues.
+1 💚	shadedclient	19m 35s		patch has no errors when building and testing our client artifacts.
			_ Other Tests _
+1 💚	asflicense	0m 38s		The patch does not generate ASF License warnings.
		92m 30s

Subsystem	Report/Notes
Docker	ClientAPI=1.41 ServerAPI=1.41 base: https://ci-hadoop.apache.org/job/hadoop-multibranch/job/PR-2653/1/artifact/out/Dockerfile
GITHUB PR	#2653
Optional Tests	dupname asflicense hadolint shellcheck shelldocs
uname	Linux 2228dcf032b9 4.15.0-126-generic #129-Ubuntu SMP Mon Nov 23 18:53:38 UTC 2020 x86_64 x86_64 x86_64 GNU/Linux
Build tool	maven
Personality	dev-support/bin/hadoop.sh
git revision	trunk / e40f99f
Max. process+thread count	514 (vs. ulimit of 5500)
modules	C: U:
Console output	https://ci-hadoop.apache.org/job/hadoop-multibranch/job/PR-2653/1/console
versions	git=2.25.1 maven=3.6.3 shellcheck=0.7.0 hadolint=1.11.1-0-g0e692dd
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[GauthamBanasandra]

@aajisaka could you please review this PR?

[aajisaka]

+1