Use conda environment for RTD builds

[umesh-timalsina]

Self explainatory.

[codecov]

Codecov Report

Merging #544 (536fc93) into master (7e21868) will increase coverage by 0.00%.
The diff coverage is 100.00%.

❗ Current head 536fc93 differs from pull request most recent head 32f12e6. Consider uploading reports for the commit 32f12e6 to get more accurate results

@@           Coverage Diff           @@
##           master     #544   +/-   ##
=======================================
  Coverage   90.03%   90.03%           
=======================================
  Files          51       51           
  Lines        3944     3945    +1     
=======================================
+ Hits         3551     3552    +1     
  Misses        393      393           

Impacted Files	Coverage Δ
gmso/core/angle.py	100.00% <ø> (ø)
gmso/core/atom.py	92.95% <ø> (ø)
gmso/core/bond.py	100.00% <ø> (ø)
gmso/core/dihedral.py	100.00% <ø> (ø)
gmso/core/dihedral_type.py	100.00% <ø> (ø)
gmso/core/improper_type.py	100.00% <ø> (ø)
gmso/external/convert_mbuild.py	98.76% <ø> (ø)
gmso/external/convert_openmm.py	100.00% <ø> (ø)
gmso/formats/gsd.py	94.44% <ø> (-0.07%)	⬇️
gmso/formats/xyz.py	97.14% <ø> (ø)
... and 2 more

---

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[lgtm-com]

This pull request fixes 1 alert when merging 54887b9 into aedc526 - view on LGTM.com

fixed alerts:

• 1 for Unused import

[lgtm-com]

This pull request fixes 1 alert when merging 6d18a15 into aedc526 - view on LGTM.com

fixed alerts:

• 1 for Unused import

[lgtm-com]

This pull request fixes 1 alert when merging e073aab into aedc526 - view on LGTM.com

fixed alerts:

• 1 for Unused import

[lgtm-com]

This pull request fixes 1 alert when merging c79155f into aedc526 - view on LGTM.com

fixed alerts:

• 1 for Unused import

[lgtm-com]

This pull request fixes 1 alert when merging 2453421 into 7e21868 - view on LGTM.com

fixed alerts:

• 1 for Unused import

[lgtm-com]

This pull request fixes 1 alert when merging 32f12e6 into 7e21868 - view on LGTM.com

fixed alerts:

• 1 for Unused import