fix: small fixes for Makefile

[betodealmeida]

SUMMARY

2 small fixes for our Makefile:

1. source venv/bin/activate doesn't work within the Makefile, since source is a shell builtin. I fixed it by doing . venv/bin/activate instead.
2. Superset only runs on Python 3.7 or 3.8 (some deps fail to install on 3.9). I added a check for the version.

BEFORE/AFTER SCREENSHOTS OR ANIMATED GIF

N/A

TESTING INSTRUCTIONS

$ make venv

ADDITIONAL INFORMATION

• Has associated issue:
• Changes UI
• Includes DB Migration (follow approval process in SIP-59)
  • Migration is atomic, supports rollback & is backwards-compatible
  • Confirm DB migration upgrade and downgrade tested
  • Runtime estimates and downtime expectations provided
• Introduces new feature or API
• Removes existing feature or API

[hughhhh]

🚀

[codecov]

Codecov Report

Merging #15670 (9c66817) into master (351d3c5) will increase coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master   #15670      +/-   ##
==========================================
+ Coverage   76.73%   76.75%   +0.01%     
==========================================
  Files         984      984              
  Lines       51713    51702      -11     
  Branches     6978     6977       -1     
==========================================
- Hits        39684    39683       -1     
+ Misses      11805    11795      -10     
  Partials      224      224              

Flag	Coverage Δ
javascript	71.51% <ø> (+0.02%)	⬆️
mysql	81.55% <ø> (ø)
postgres	81.57% <ø> (ø)
python	81.66% <ø> (ø)
sqlite	81.19% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
...rontend/src/SqlLab/components/QueryTable/index.jsx	70.31% <0.00%> (+8.97%)	⬆️

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