fix doc ci by pinning jinja version

[xinjli]

Doc CI is failing now because of a Jinja2 version issue.

Explanation

• Our current sphinx version explicitly depends on environmentfilter from jinja2, which was depreciated recently.
• Trying to import this module without pinning to an old jinja2 would cause the ImportError and fail the doc CI process.
• More details can be seen in this issue: Jinja 2 imports have been deprecated sphinx-doc/sphinx#10291

Fix

The latest sphinx (>4.0.5) seems to already handle this issue, but because we are depending on an old version (2.1.2), we still have this issue. Maybe we should consider upgrading sphinx version in the future.

• As suggested in the previous issue, pinning jinja to an old version should solve this issue.
• I have verified that this import error did not happen after pinning in my local environment.

[codecov]

Codecov Report

Merging #4239 (e5bfb06) into master (c4aba12) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #4239   +/-   ##
=======================================
  Coverage   80.71%   80.71%           
=======================================
  Files         453      453           
  Lines       39574    39574           
=======================================
  Hits        31943    31943           
  Misses       7631     7631           

Flag	Coverage Δ
test_integration_espnet1	67.13% <ø> (ø)
test_integration_espnet2	50.05% <ø> (ø)
test_python	67.16% <ø> (ø)
test_utils	24.45% <ø> (ø)

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[sw005320]

Thanks!