[bugfix] Add importlib_metadata to requirements

[victorusu]

While working on the improved spack recipe for ReFrame, I got the following error:

==> Installing py-pip
...
==> Installing reframe
...
$ ml reframe
$ reframe -l
Traceback (most recent call last):
  File "/users/hvictor/spack-victor/opt/spack/linux-centos8-broadwell/gcc-8.3.1/reframe-bootstrap-z2zqkak5qb5kepxr2p4pyryrbck4ma2c/external/jsonschema/__init__.py", line 31, in <module>
    from importlib import metadata
ImportError: cannot import name 'metadata'
During handling of the above exception, another exception occurred:
Traceback (most recent call last):
  File "/users/hvictor/spack-victor/opt/spack/linux-centos8-broadwell/gcc-8.3.1/reframe-bootstrap-z2zqkak5qb5kepxr2p4pyryrbck4ma2c/bin/reframe", line 18, in <module>
    import reframe.frontend.cli as cli  # noqa: F401, F403
  File "/users/hvictor/spack-victor/opt/spack/linux-centos8-broadwell/gcc-8.3.1/reframe-bootstrap-z2zqkak5qb5kepxr2p4pyryrbck4ma2c/reframe/__init__.py", line 26, in <module>
    from reframe.core.pipeline import *     # noqa: F401, F403
  File "/users/hvictor/spack-victor/opt/spack/linux-centos8-broadwell/gcc-8.3.1/reframe-bootstrap-z2zqkak5qb5kepxr2p4pyryrbck4ma2c/reframe/core/pipeline.py", line 26, in <module>
    import reframe.core.runtime as rt
  File "/users/hvictor/spack-victor/opt/spack/linux-centos8-broadwell/gcc-8.3.1/reframe-bootstrap-z2zqkak5qb5kepxr2p4pyryrbck4ma2c/reframe/core/runtime.py", line 14, in <module>
    import reframe.core.config as config
  File "/users/hvictor/spack-victor/opt/spack/linux-centos8-broadwell/gcc-8.3.1/reframe-bootstrap-z2zqkak5qb5kepxr2p4pyryrbck4ma2c/reframe/core/config.py", line 10, in <module>
    import jsonschema
  File "/users/hvictor/spack-victor/opt/spack/linux-centos8-broadwell/gcc-8.3.1/reframe-bootstrap-z2zqkak5qb5kepxr2p4pyryrbck4ma2c/external/jsonschema/__init__.py", line 33, in <module>
    import importlib_metadata as metadata

[vkarak]

lgtm

[codecov-commenter]

Codecov Report

Merging #1503 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1503   +/-   ##
=======================================
  Coverage   91.65%   91.65%           
=======================================
  Files          83       83           
  Lines       13012    13012           
=======================================
  Hits        11926    11926           
  Misses       1086     1086           

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