Update license metadata and parse-conda-requirements

[alurban]

This PR makes a few bookkeeping changes:

• Update license metadata to reference GPL-3.0-or-later
• Inherit parse-conda-requirements.py directly from gwpy, rather than copying it without attribution

cc @duncanmmacleod, @jrsmith02

[codecov]

Codecov Report

Merging #145 into master will decrease coverage by 0.22%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #145      +/-   ##
==========================================
- Coverage   34.74%   34.52%   -0.22%     
==========================================
  Files          12       12              
  Lines        1497     1492       -5     
==========================================
- Hits          520      515       -5     
  Misses        977      977              

Flag	Coverage Δ
#Linux	34.52% <ø> (-0.22%)	⬇️
#python36	34.52% <ø> (ø)
#python37	34.52% <ø> (ø)
#python38	?

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Impacted Files	Coverage Δ
hveto/core.py	25.32% <0.00%> (-0.93%)	⬇️
hveto/html.py	55.84% <0.00%> (-0.57%)	⬇️
hveto/segments.py	92.50% <0.00%> (-0.18%)	⬇️

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