Fixed JSON encoding error on Google Analytics reporting

[NyanKiyoshi]

Closes #4050

Pull Request Checklist

1. Privileged views and APIs are guarded by proper permission checks.
2. All visible strings are translated with proper context.
3. All data-formatting is locale-aware (dates, numbers, and so on).
4. Database queries are optimized and the number of queries is constant.
5. Database migration files are up to date.
6. The changes are tested.
7. GraphQL schema and type definitions are up to date.
8. Changes are mentioned in the changelog.

[codecov]

Codecov Report

Merging #5004 into master will increase coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #5004      +/-   ##
==========================================
+ Coverage    91.2%   91.21%   +0.01%     
==========================================
  Files         351      351              
  Lines       21403    21404       +1     
  Branches     2044     2044              
==========================================
+ Hits        19520    19524       +4     
+ Misses       1328     1326       -2     
+ Partials      555      554       -1

Impacted Files	Coverage Δ
saleor/core/analytics.py	94.28% <100%> (+8.99%)	⬆️

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[django-queries]

Here is the report for 8301dde (NyanKiyoshi/saleor @ fix/google-analytics)
Base comparison is c3a5d79.

No differences were found. (click me)

# api.benchmark checkout
  test name                                  	left count 	right count	duplicate count
  -------------------------------------------	-----------	-----------	---------------
  add billing address to checkout            	         34	         34	             20
  add shipping to checkout                   	          7	          7	              0
  checkout payment charge                    	         10	         10	              0
  complete checkout                          	          8	          8	              0
  create checkout                            	         50	         50	             24

# api.benchmark homepage
  test name                                  	left count 	right count	duplicate count
  -------------------------------------------	-----------	-----------	---------------
  retrieve main menu                         	          5	          5	              0
  retrieve product list                      	          4	          4	              0
  retrieve secondary menu                    	          5	          5	              0
  retrieve shop                              	          2	          2	              0

# api.benchmark product
  test name                                  	left count 	right count	duplicate count
  -------------------------------------------	-----------	-----------	---------------
  product details                            	         15	         15	              3
  retrieve product attributes                	          9	          9	              0

# api.benchmark variant
  test name                                  	left count 	right count	duplicate count
  -------------------------------------------	-----------	-----------	---------------
  product variant bulk create                	         51	         51	              3
  retrieve variant list                      	         15	         15	              6

# api product sorting attributes
  test name                                  	left count 	right count	duplicate count
  -------------------------------------------	-----------	-----------	---------------
  sort product not having attribute data     	         21	         21	              0

[maarcingebala]

@NyanKiyoshi Black failed on Travis, can you fix the formatting?

[NyanKiyoshi]

Done, thanks! Looks like isort messed up with the black formatting 😉 (I run them manually)