Fixes PDBWriter and PDBReader newlines for PDB header

[RMeli]

Fixes #2324

Changes made in this Pull Request:

• Stripped PDB HEADER in PDBReader of newline trailing characters.
• Stripped PDB HEADER in PDBWriter of newline trailing characters.

I'm not sure which tests need to be added, since PDBReader and PDBWriter should already be tested.

This problem does not really concern MDAnalysis, since it can cope with PDB files containing empty lines.

PR Checklist

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

[codecov]

Codecov Report

Merging #2325 into develop will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff            @@
##           develop    #2325   +/-   ##
========================================
  Coverage    89.75%   89.75%           
========================================
  Files          173      173           
  Lines        21536    21536           
  Branches      2804     2804           
========================================
  Hits         19329    19329           
  Misses        1615     1615           
  Partials       592      592

Impacted Files	Coverage Δ
package/MDAnalysis/coordinates/PDB.py	90.63% <100%> (ø)	⬆️

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[richardjgowers]

Thanks @RMeli