Replace deprecated imp

[yuxuanzhuang]

Fixes #2897

Changes made in this Pull Request:

• replace deprecated imp with types to mock chemfiles when building docs.

PR Checklist

• Tests?
• Docs?
• CHANGELOG updated?
• Issue raised/referenced?

[codecov]

Codecov Report

Merging #2904 into develop will decrease coverage by 0.02%.
The diff coverage is 100.00%.

@@             Coverage Diff             @@
##           develop    #2904      +/-   ##
===========================================
- Coverage    92.89%   92.86%   -0.03%     
===========================================
  Files          187      187              
  Lines        24587    24587              
  Branches      3192     3192              
===========================================
- Hits         22840    22833       -7     
- Misses        1701     1708       +7     
  Partials        46       46              

Impacted Files	Coverage Δ
package/MDAnalysis/coordinates/chemfiles.py	94.88% <100.00%> (ø)
package/MDAnalysis/due.py	66.66% <0.00%> (-8.34%)	⬇️
package/MDAnalysis/coordinates/base.py	94.64% <0.00%> (-0.49%)	⬇️

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[orbeckst]

lgtm