Improve test compatibility with new pyproj

[avalentino]

Recent pyproj introduced a different naming for the web-mercator projection.
The patch takes into account of the different naming and make all tests pass properly also with new versions of pyproj.

• Tests added
• Tests passed
• Passes git diff origin/master **/*py | flake8 --diff

Edit: see also https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=949274

[avalentino]

Rebased on the current master

[codecov]

Codecov Report

Merging #304 into master will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #304   +/-   ##
=======================================
  Coverage   92.65%   92.65%           
=======================================
  Files          45       45           
  Lines        9134     9135    +1     
=======================================
+ Hits         8463     8464    +1     
  Misses        671      671           

Impacted Files	Coverage Δ
pyresample/test/test_utils.py	98.42% <100.00%> (+<0.01%)	⬆️

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[coveralls]

Coverage increased (+0.0008%) to 92.655% when pulling 524048a on avalentino:pyproj-compat into bbd8054 on pytroll:master.

[djhoese]

Thanks!