Install Python Dev Dependencies to Virtual Environment

[U8NWXD]

Overview

1. This PR fixes or fixes part of #[n/a].
2. This PR does the following: Instead of installing Python dev dependencies to oppia_tools, this PR changes our installation code to install them to the user's virtual environment. This lets us pin our transitive dev dependency versions using a requirements_dev.txt file much like we do with a requirements.txt file for our production python dependencies.

Essential Checklist

• The PR title starts with "Fix #bugnum: ", followed by a short, clear summary of the changes. (If this PR fixes part of an issue, prefix the title with "Fix part of #bugnum: ...".)
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• The PR is made from a branch that's not called "develop".

Proof that changes are correct

I have verified that this PR works correctly by confirming that installing Oppia from scratch works macOS, and I have worked with vojtechjelinek and kevintab95 to test on Linux.

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[oppiabot]

Hi @U8NWXD, can you complete the following:

1. The karma and linter checklist has not been checked, please make sure to run the frontend tests and lint tests before pushing.
   Thanks!

[oppiabot]

Hi, @U8NWXD, this pull request does not have a "CHANGELOG: ..." label as mentioned in the PR pointers. Assigning @U8NWXD to add the required label. PRs without this label will not be merged. If you are unsure of which label to add, please ask the reviewers for guidance. Thanks!

[oppiabot]

Hi @U8NWXD, there is a new change in develop which needs to be in your PR. Please update your branch with the latest changes in develop. For instructions, refer to this link. Thanks!

[oppiabot]

Hi @U8NWXD, the build of this PR is stale and this could result in tests failing in develop. Please update this pull request with the latest changes from develop. Thanks!

[oppiabot]

Hi @U8NWXD, the build of this PR is stale and this could result in tests failing in develop. Please update this pull request with the latest changes from develop. Thanks!

[oppiabot]

Hi @U8NWXD, I'm going to mark this PR as stale because it hasn't had any updates for 7 days. If no further activity occurs within 4 days, it will be automatically closed so that others can take up the issue.
If you are still working on this PR, please make a follow-up commit within 4 days (and submit it for review, if applicable). Please also let us know if you are stuck so we can help you!

[oppiabot]

Hi @U8NWXD, can you complete the following:

1. The karma and linter checklist has not been checked, please make sure to run the frontend tests and lint tests before pushing.
   Thanks!

[oppiabot]

Hi, @U8NWXD, this pull request does not have a "CHANGELOG: ..." label as mentioned in the PR pointers. Assigning @U8NWXD to add the required label. PRs without this label will not be merged. If you are unsure of which label to add, please ask the reviewers for guidance. Thanks!

[U8NWXD]

@seanlip I've addressed your comments. PTAL

[vojtechjelinek]

LGTM!

[oppiabot]

Unassigning @vojtechjelinek since they have already approved the PR.

[U8NWXD]

@seanlip PTAL

[seanlip]

Thanks @U8NWXD! LGTM, just found one typo. Thanks for removing all the import path messiness!

[oppiabot]

Hi @U8NWXD, this PR is ready to be merged. Please address any remaining comments prior to merging, and feel free to merge this PR once the CI checks pass and you're happy with it. Thanks!